I don't have all the usage stats for what's been changing ever since FloonDig became the meta, so I probably wasn't very clear or informed about this. I do, however, still believe diversity is all about adaption to the metagame. Without Diglett, we wouldn't see these innovations.
Here's a question: What about the innovations that Diglett is suppressing? Adaptation is all well and good, and there needs to be a problem for a solution to appear, but what if Diglett really is hampering diversity and the small number of checks rising to meet it are smaller than the group of Pokemon that could be viable without it around?
I disagree with Abra here. In my personal experience, Pumpkaboo-XL can also check Diglett very effectively, because it can even break the Sub set and doesn't care about Focus Sash. In the context of just Focus Sash users, though, Abra is actually harder to check than Diglett because it doesn't take damage from hazards or burns, while Diglett does. Throw in a Scarf pivot like Chou or Mag, and you're golden. These Pokemon lure in Diglett easily and have no problems taking it out with any type of hazard. Diglett is also forced out by common Grass-types like Ferroseed and Cottonee, and even the less common Lileep, which has grown in popularity due to DigFloon's presence, and isn't even "niche" in the context of "not fitting on teams" or "being sub-optimal" either. It's a solid Pokemon that now has a more substantial role because it can force out Diglett; that's diversity.
When is Pumpkaboo ever going to be against Diglett? Diglett can't trap Pumpkaboo and it resists Ground, so Pumpkaboo can only force out Diglett. That seems fine for an ordinary Pokemon, but tell me, how is Pump getting in against Diglett? The only way is after a KO, because Diglett gets to pick its fights. That is the whole reason why people complain about it: by the time you can force it out, it has done its job and picked up a KO. Forcing out Diglett is not a victory like forcing out Abra is, because you are guaranteed to lose a Pokemon. Ferroseed and Cottonee are the same way, as it doesn't matter if Diglett is forced out. As for Lileep, it really isn't a solid Pokemon as you say, because if it were, it would already have been used more before FloonDig came into play. Lileep is bad compared to 5th gen Lileep, as the lack of perma-sand, the rise of Knock Off, Pawniard, and the continuing dominance of Fighting-types makes it much more of a liability. Archen is way better at checking FloonDig than Lileep is, since it has higher Attack, immunity to Ground, the same reliable recovery in Roost, the same utility with Stealth Rock + the possibility of Defog, and a way to beat Fighting-types with Acrobatics. Naturally, Lileep does have some other benefits like being able to flawlessly check Chinchou and better bulk in general, but there's a reason why Lileep has fallen off and it didn't come back to take on FletchDig.
You misunderstood my context about Abra. I was referring to fast and frail attackers, which I clearly said in my previous post. Diglett is not immune to hazards, but it isn't bothered much by them either. Stealth Rock takes away 1 HP, while Spikes and Toxic Spikes are pretty rare, but still don't really bother it because it's not taking hits anyway and already loses to most forms of priority.
The VGC metagame is a real mess right now, so I wouldn't compare that metagame, which is fast paced and full of Ubers along with other broken mons, to ours, which is very stable and very diverse. Our metagame is great, and Diglett contributes to this by being a hurdle that teams have to overcome. Diglett's main role is a revenge killer, and it acts how a revenge killer should. Just because it happens to revenge kill some Pokemon that people really like using that were more viable before Diglett's rise in usage doesn't mean it stops you from running those Pokemon entirely. Diglett is a big threat, but it kind of stops there. It's not like other broken Pokemon we've banned in the past: it has no bulk, it is only used specifically to trap threats to the cores it's used in, and it's incredibly hard to switch in. Diglett isn't something that sweeps everything or is unkillable, it exists purely as a support Pokemon, and has little to no versatility outside of that, barring an SR and Sub set that can be played around.
So you say a revenge killer should be able to always pick which targets it will eliminate with the only viable counterplay being "don't get trapped"? I don't think that's how revenge killers should work. It's not that Diglett removes Pokemon people "really like using", it's that it removes Pokemon without giving the opponent the option to switch out. Also, the fact that it is unlike other banned Pokemon is entirely irrelevant. We don''t compare suspects to previously banned Pokemon, we compare them to the metagame around them and whether or not they are too much to handle in one way or another. Speaking of that, support Pokemon are absolutely banworthy if the support they give makes a wide variety of other Pokemon significantly better. Diglett doesn't need to be super strong and super bulky to be banworthy.
Also, I've still seen plenty of those barring Stunky and Honedge, the latter of which wasn't really that good to begin with save as a Meditite counter back in the day, so I wouldn't say Diglett crosses all of those Pokemon off the map. Sun is still a very strong playstyle, and Croagunk and Larvesta can still serve purposes on teams even if Diglett can trap them. Skrelp is definitely a Pokemon that suffers from Diglett's position in the meta, and it's probably the best example of all of those you listed. The thing is, Skrelp's problem isn't limited to just Diglett. In the context of trapping, which I believe is the main problem with Diglett, not its supposed limits on teambuilding, Gothita does the same thing. Gothita also traps Croagunk and a plethora of other Fighting-types that Diglett can't check while boasting better coverage than Diglett anyways. Now I'm not saying that Gothita is broken, either, or that it limits teambuilding. Diglett's EQ isn't even guaranteed to OHKO Skrelp, so if you play smart, you're not going to have as big of a problem as you'd think. Skrelp is also just countered by pretty much every Pokemon that knows Earthquake. Pinning that on Diglett is pretty silly, in my opinion.
In my opinion, you have a very wrong understanding of Skrelp. It is bulkier than you think, and definitely doesn't lose to anything with Earthquake. In fact, the only EQ that actually OHKOes Skrelp at full HP is LO Diglett. Not even max Attack Eviolite Drilbur can do it. The fact that you say this really makes me question if you really understand what you're talking about.
In any case, the problem is not that those Pokemon are entirely unusable, and I don't believe I ever said that. I simply meant that Diglett impacts their viability, which was a response to you saying that Diglett adds diversity through checks rising from obscurity to defeat it. Gothita as a trapper not at all similar to Diglett, because it is not only much weaker, but also more predictable than Diglett is. As I said before, Skrelp is OHKOed by LO Diglett's EQ, but Gothita only does 80%, thus allowing it to get OHKOed by Hydro Pump 56% of time, or all the time after SR. Gothita has this problem across the board, being unable to KO most Fighting-types besides Croagunk, it can't OHKO most things with its coverage moves, and it has very little utility outside of Trick. It's also locked into the move it chooses as a result of its Scarf, leaving it entirely open to being taken advantage of after it gets a KO. The same can happen to Diglett occasionally, but not as often since it can change moves and has access to moves like Substitute and Memento. Gothita is not at all on the same level as Diglett.[/quote]