Maybe it's because I'm a Hoenn baby, but the physical-special split is one of the worst things to happen to Pokemon.
Yes, there are some Pokemon that get screwed over pre-split (ex. Sneasel and Gengar have both STABs in their non-dominate offensive stat), but both of my examples still ended up being viable options in their RSE tiers they ended up in.
Sneasel was in UU, sure it was a top pick in UU but still that feels like giving it faint praise. And while Sneasel dropped to NU in Gen 4, that's cause Weavile came along (and thus started Sneasel's reign of terror on Little Cup, forever for each gen to be allowed back in as long as it promised not to cause trouble only to be kicked out again cause people really expected a Dark-type not to cause trouble).
Gengar was able to remain OU because it had very high Super Attack & Speed plus learned a lot of Special moves, really had nothing to do with Type.
And pre-split really makes some Pokemon unique, and gives some Pokemon niches that I would've liked to have seen further explored, instead of making them all homogenous. My core argument, beyond simplicity, is that pre-split gave individual Pokemon their own identities when compared to others of their type combination, and, as a result, forced them to be thoughtfully created to make them distinct.
I'm not quite sure about that. There's only so things a Special Water-type could do to try and make itself unique, but at the end of the day a lot of people are probably still gonna pick the few Special Water-type that hits hard (and hit fast). Okay, so maybe the Water-type can be bulky instead, but yet again once there's a few bulky Water-types only the "top ones" are gonna get used. Overall, as more and more Pokemon are added, I feel this is a moot point to blame on the "Physical-Special Split" as competition would force only a handful of Pokemon to be the top picks no matter if all Pokemon have a niche; people pick what works. If anything the "Physical-Special Split" gives each Type some breathing room as it opens up at least a few more categories they can have Pokemon based on thus a better chance for those Pokemon becoming top picks where they otherwise wouldn't have a chance.
The split allows them to design Pokemon that get to have their cake and eat it too, whereas without it, deciding your Pokemon involves a deliberate series of tradeoffs because barely any get to eat their cake.
... Wait, what? So you're saying having more choice is bad and pre-split was good because it only allowed Types to have even a more select amount of top choices than it does now? I feel that's kind of like splitting hairs.
In conclusion, I believe that the physical/special split does not actually support the thoughtful creation of Pokemon, and instead results in Pokemon creation symbolic of the current state of Pokemon: creatures rushed out the door in a manner that lacks intentionality or focus. As I have argued with this analysis of Dark-types, I have shown that Pokemon that are unable to effectively use their STABs due to their statspread have other features or traits that allow them to still function as unique creatures, which is a nuance that has been lost in post-split games due to the increased homogeneity the split creates.
Let's look at the other Dark-types for a second here:
Darkrai: HA HA HA! "Would Darkrai still have been good", that's a good one...
Liepard: Pretty much the cat version of Mightyena. However it is notably faster with Atk & SpA equal but less bulky. Liepard's gimmick is about being a fast attacker like Sneasel, its Abilities further re-enforcing this (it even gets Prankster as a Hidden Ability). I think it would have operated just fine in pre-split as it does now.
Zoroark: It's pretty much a better Liepard. Would be fine, and it's main gimmick doesn't even involve its Type but rather tricking its opponent into thinking it's another Pokemon (or rather, another Type probably looking to be hit by something it resists or is immune to).
Thievul: Fun fact I just learned, despite having a mostly Physical movepool, Thievul is a Special Attacker, what? Thievul I think is a less fast but more bulky Liepard, it's main gimmick I think it having access to all sorts of Dark-type moves which does various effects instead of focusing on strict damage (or rather finding other ways to do more damage than relying on its own stats).
Alolan Raticate: It's more bulky than normal Raticate... which doesn't really help as by making it part Dark it has a quad weakness to Fighting a new weaknesses to Fairy and Bug. It's Abilities also don't really help it so I guess you can say this one is a dud, but being it's an Alolan Variant I think its purpose was more to show off the new mechanic than be a good Pokemon.
Obstagoon: So it is a Physical Attacker so would very much take the most advantage from the split... except doing straightforward damage isn't its gimmick. Its a counterattacker, heck after Obstructing the opponent it'll probably follow-up with a Physical move of another Type like an elemental punch, Close Combat, Seed Bomb, or X-Scissor (or if this was pre-Split, just Close Combat or X-Scissor, maybe a Shadow Claw).
Honchkrow: Okay, I think we now have our first Pokemon who would probably take the most advantage of the split, though to be fair it's Special Attack isn't bad and Flying was Physical so even pre-split I think Honchkrow would have done fine, just limited in what it could do cause its a bird.
Weavile: Okay, NOW we have a Dark-type that couldn't really operate as good as it does without the split. And because of that it's one of top Physically Offensive Dark/Ice-types thanks to access to Pursuit, Knock Off & Ice Shard which aren't even that of moves but its Attack is high enough that it makes them work. But what if it existed pre-split, well I think it would just be a faster but frailer Absol as both get similar moves; so if Absol was fine then Weavile would have also been fine, but not nearly as good as it is now cause of the niches the split have given it.
Scrafty: Sort of relies on the split... but it's also a Fighting-type so not really. Heck, I don't think Scrafty really relies on any Dark-type moves now aside Knock Off and that's just cause Knock Off has a useful effect. That said Scrafty has other issues holding it back like it being a defensive Pokemon but has low HP and its Abilities don't really support it as a tank.
Bisharp: I think it's on the same boat as Scrafty aside what's holding it back. Steel was Physical so would still have Iron Head. Also, if a Dark-type using using Knock Off it's likely using Foul Play if it doesn't have a high offense stat it can rely on and I'd imagine that would have been the case for many Dark-types had the move existed pre-split.
Mandibuzz: Mainly a defensive Pokemon so its tactics would involve indirect damaging means and providing support.
Hydreigon: HA HA HA!
Malamar: A bit tricky as, while it is a Physical Attacker, it's main gimmick is about inflicting itself with status debuffs which then Contrary turns into buffs (while ideally doing the opposite for opponents). It does learn Superpower so wouldn't be totally useless pre-split, and I'd imagine it would have Stored Power on the backburner after getting enough stat increases. Any Dark-type move honestly would be utility... Knock Off.
Yveltal: Would still stuck out your life force.
Guzzlord: It would do as fine as it does now, which isn't really much but don't see how it existing pre-split would change that.
Grimmsnarl: Now Grimmsnarl is an interesting case as I'm sure Fairy would have been Special back in the day. Grimmsnarl is a Physical Attacker so neither STAB would be going off its highest stat, that said its Special Attack is far from bad. Also it does learn a decent amount of Fighting moves plus a few notable options like Shadow Claw, Stomping Tantrum, and Leech Life. So I think we got another Absol situation here but I think Grimmsnarl would be able to play as a mixed attacker getting the best of both worlds.
Dada Zarude Sandstorm: Zarude I ain't so sure about. It is primarily Physical and wouldn't really use its Special which pre-split are where both of its STABS are. However it does get a few good Physical moves still and one of its gimmick is healing itself and its allies (and it have a nice amount of bulk in HP, Defense, and Special Defense). So pre-split I think it would still have fine, but it does really benefit more from the split.
Alolan Muk: Oddly the benefit of Alolan Muk being part Dark is that it loses its Psychic weakness, now only being weak to Ground, and becomes immune to Psychic. Pre-split that's all it would have gained, but post-split it now also gains strong Physical Dark-type STABs.
Mega Gyarados: Gyarados was like Gengar, didn't have any STAB to rely on but was so powerful the Physical moves it did have was good enough, so Mega Gyarados would have been just as fine pre-split. And, like, I guess Mega Gyarados appreciates Crunch but I think it more appreciates Waterfall being Physical.
Skuntank: Now this is an odd one. Being Poison-type it does get to use that bigger Attack for STAB, though Stuntank's gimmick seems more about punishing the opponent for attacking it via Aftermath and until then just trying to inflict it with Poison and status debuffs with Venom Drench. Heck, Stuntank has two other ways of fainting itself: Explosion (that would have gone off Attack eitherway) and Memento. It also is only weak to Ground and immune to Psychic much like Alolan Muk. Overall, not sure if the split helps it that much but doesn't seem to have hurt it.
Spiritomb: Don't think it would have been any different.
Drapion: I think we have a combination of the Absol effect & and the Type Chart benefits of Poison/Dark. Drapion benefits from the split no doubt, but being Poison-type and evolving from a Bug-type means it does get those moves upon other strong Physical moves (such as Ground and Ghost). And even if it was pre-split no one would complain about having Knock Off.
Krookodile: I think Krookodile is a case where without the Split it would have no reason being Dark-type. Dark/Ground has a bunch of popular weaknesses with not so great resistances. While it would have Ground (and Rock) to use its Attack stat in among others, Dark gives it no benefit without it having moves that go off its Physical stat.
Greninja: I think would pretty much been the same as it is now, lol.
Pangoro: Another Fighting/Dark, though Pangoro does really focus on its Attack stat so Dark does feel like its not that helpful here if it didn't have Psychical Dark moves.
Hoopa Unbound: HA HA HA!
Incineroar: Oh geez, I think without the Split they would have made Incineroar Fire/Fighting so that alone I'm approving of the Physical/Special split! But if they did keep it Fire/Dark, eh, wouldn't be as good I guess though would still have Physical moves open for it to use, plus it's Special Attack isn't that bad for STAB. It also has quite the number of resistances too, though a few popular weaknesses evens that out.
Morpeko: Well they probably would have made it a Special Attacker in the first place, but otherwise I don't think much else about it would have changed.
Urshifu Single Strike: Yeah, without the Split I feel Urshifu Single Strike wouldn't be Dark-type (heck, I don't think Rapid Strike would have been Water either). They rely on the Split for their secondary Type to hold up (though they probably would've gotten different secondary Types altogether, like Single Strike being Fighting/Ground and Rapid Strike being Fighting/Flying).
So, yeah, while certainly some Physical Dark-types would have loss effectiveness if they existed pre-split, I think they would still have been just as much functional as you said with the Dark-types you mentioned. However what the Split did was allow these Pokemon, plus the Dark-types you mentioned, just get more utility. However, maybe Dark-type is just a bad example, like before the split it always felt odd it was Special and then when the Split happened almost all Dark-type moves became Physical and they specifically had to create Dark Pulse so that Special Dark-types would have a STAB to use!
BUT, with all that said, I maybe wouldn't mind a special battling mode that reverts all Types into either being Physical or Special like the pre-split (Fairy being Special as I mentioned). Probably too powerful to be a "Room" move, though I wonder if maybe it wouldn't be a bad idea for a held item...
I don't particularly agree with the sentiment that we need a physical HP.
Not like it matters anymore as they removed Hidden Power. Though if they did bring HP back I think they should just have a way to change it from being either Special or Physical, though maybe also lower its Power to like 50.
But the physical/special split is one example of a different kind of compression: so many Pokemon have such amazing and useful options, that it makes teambuilding more restrictive.
That's sadly a problem with the core battling UI being essentially the same as it was since Gen I. I do think GF should update the battling UI to somehow allow Pokemon to at least access to more moves during a battle, even if they make it so after choosing 4 moves you then get locked into just those 4 moves.