I mean you're ignoring the possibility of tera ghost on any physical wall if you really want security. But also it's possible with tactics like fast Corv/Skarm with ID BP to cover for SD Luna. Regarding your remark about BO/Balance, running a single fast pokemon to offensively check it is nowhere near enough. Not at a high level anyways. As for Specs Kyurem I'm not going to argue much against it simply because I don't think its presence is desirable long term either, but also because one man that is questionable avoided a ban doesn't mean something else questionable isn't banworthy. You're also oversimplifying it. If your Tera Dragon mon isn't bulky enough, it's still capable of being blown up by +2 Ogerpon, especially if factoring in its tera. Or they'll just get beaten by encore sets. Having to depend on Tera because there's a shortage of checks (in this case defensive) is usually a bad sign of balance or rather a lack of balance of a pokemon. We've seen it multiple times this gen.
I think that having to slap on tera ghost for a mon isn't really the best thing in the world to deal with ursaluna.
252+ Atk Guts Ursaluna Fire Punch vs. +2 252 HP / 160+ Def Skarmory: 88-104 (26.3 - 31.1%) -- guaranteed 4HKO
While they can barely beat it, skarm/corv do have to be careful of coming in as otherwise it will be a 2hit ko.
Honestly, yes, most of the time a faster offensive mon is enough to check waterpon.
252 Atk Wellspring Mask Ogerpon-Wellspring Ivy Cudgel vs. 0 HP / 0 Def Enamorus: 249-294 (86.1 - 101.7%) -- 12.5% chance to OHKO
Enamorus is extremely frail, and even in a pinch it can check waterpon, if it takes any damage than it may be screwed, but that's not the point. The point is that enamorus puts it in range of most priority moves.
252 SpA Enamorus Moonblast vs. 0 HP / 4 SpD Ogerpon-Wellspring: 165-195 (54.8 - 64.7%) -- guaranteed 2HKO
So yeah, I would say that it usually enough for it. Fast mon + priority, which every non stall team should have I believe. I think there was a pinkacross video about it, so imma just use that as a source.
+2 252 Atk Wellspring Mask Ogerpon-Wellspring Ivy Cudgel vs. 0 HP / 0- Def Dragapult: 261-308 (82.3 - 97.1%) -- guaranteed 2HKO
Minus defense btw, with a mon that is frail. I went through the tera type index (which has to be updated btw), and picked out mons that were kinda frail that use tera dragon.
Manaphy
+2 252 Atk Wellspring Mask Ogerpon-Wellspring Ivy Cudgel vs. 0 HP / 4 Def Tera Dragon Manaphy: 184-217 (53.9 - 63.6%) -- guaranteed 2HKO after Leftovers recovery (Lmao, I forgot it is water type to begin with, but still)
+2 252 Atk Wellspring Mask Tera Water Ogerpon-Wellspring-Tera Ivy Cudgel vs. 0 HP / 4 Def Manaphy: 246-290 (72.1 - 85%) -- guaranteed 2HKO after Leftovers recovery
Volcarona
+2 252 Atk Wellspring Mask Ogerpon-Wellspring Ivy Cudgel vs. 252 HP / 104 Def Tera Dragon Volcarona: 227-267 (60.6 - 71.3%) -- guaranteed 2HKO
+2 252 Atk Wellspring Mask Tera Water Ogerpon-Wellspring-Tera Ivy Cudgel vs. 252 HP / 104 Def Tera Dragon Volcarona: 303-357 (81 - 95.4%) -- guaranteed 2HKO
So, yeah, I don't think that waterpon once it gets up an sd its "gg, shake my hand". Even decently frail mons can tera dragon and still take a hit, even if it sds and teras. So imagine bulky resists.
If they are running encore, guess what? They can't use play rough. Meaning that they now are countered by any decently fast dragons types, which there are a decent amount of in this meta. Sure, if the only way to defensively counter a mon is to tera, I would agree, however, mons in there base typing can take a hit from it, even at +2
I'll give you a list:
Corviknight
Dondozo
Dragapult
Dragonite
Gouging Fire
Kingambit
Kyurem
Raging bolt
Roaring Moon
Serperior
Skarmory
Zamazenta
Now, of course play rough can be difficult for at lot of these, but only kyurem, corviknight, gouging fire and skarmory don't have either priority or outspeed waterpon. I would say that that is sufficient counterplay. This is all assuming that it gets to +2, which it shouldn't get to for free. It should get up an sd, then get threatened out by one of these revenge killers.
Again, you're greatly oversimplifying the process. Wellspring gets many opportunities against slower teams unless you run like 2 big checks and a soft check on top (like a Dragapult for offensive resists, which btw gets 2HKOd by Ivy Cudgel if a crit happens), but that's just showing the problem of having to dedicate so much teambuilding space to handle it.
I would say that the mons that you need to check for waterpon are mons that commonly are used regardless of it being there. Again, you maybe need 1 hard check and 1 soft check, and saying that a mon does not counter something if it gets a crit I don't think really matters. Sure, ivy cudgel has a higher crit rate, so that can be dumb, but sometimes crits happen and that is unfortunate.
Again, I'm saying this mainly from experience from placing formats where Ferro is legal with Wellspring and while Ferro can help against some variants, it's weak to sets like Superpower and Encore, with the latter making Ferro set up bait the moment it clicks leech seed or stealth rock.
I'll briefly touch on the defensive stuff you listed before closing out though. Tera Dragon Gliscor has two big issues, one of which is the newfound hazard vulnerability and two is it now becoming weak to Valiant which it could formerly check decently. Gliscor is also Encore bait and thus, set up food. Mandibuzz is the same, as are Skarmory and Corviknight (the instant they click any passive move they lose). Hydrapple needs heavy phys investment or it dies to Play Rough (and is stall mostly so it's not easy to put on Balance or BO, which adds to the issue). Amoonguss is a terrible mon post sleep ban so it's not even worth talking about, and Toxapex is also not really good outside stall and is, again, Encore food. And Wo Chien sucks.
You say fast meta, but a big reason it is this way (and also prone to match up fishiness) is because there's so much to account for that it can really be difficult to build those other teams, and pokemon like Ogerpon-W greatly contribute to that. You also say " The issue is that people don't realise that just because a mon is hard to defensively check, doesn't mean it is hard to offensively check. If we went with that, then crawdaunt should be banned" which, no offense, comes off as a bit disingenous. Pokemon like Crawdaunt are possible to limit even with slower teams because it's so absurdly frail, difficult to bring in, and vulnerable to all manner of chip on top of offensive counterplay. Slower teams don't have nearly the same luxury vs Ogerpon without going out of their way with bringing checks because of its power and speed.
Okay, so if ferro is having to face superpower waterpon, than that is definetely an issue. However, if it happnes to be encore than guess what? You switch in your dragon type. If it is superpower, than that is unfortunate, but now you know that your dragon type is better against it.
Gliscor is good against one valiant set, sd, without moonblast which I think is rarer. Calm mind valiant destroys gliscor, it can 2hit ko gliscor easily. Gliscor may be encore bait, but you can easily switch out. And if it is encore, than guess what?
+6 252 Atk Wellspring Mask Ogerpon-Wellspring Power Whip vs. 244 HP / 248+ Def Tera Dragon Gliscor: 274-323 (77.8 - 91.7%) -- guaranteed 2HKO after Poison Heal
+6 252 Atk Wellspring Mask Tera Water Ogerpon-Wellspring-Tera Ivy Cudgel vs. 244 HP / 248+ Def Tera Dragon Gliscor: 305-359 (86.6 - 101.9%) -- 12.5% chance to OHKO
You have to get to +6 and tera in order to ohko gliscor, and gliscor can easily get a toxic/poison jab off, which the stall team can take advanatage of.
Mandibuzz gets to id up, and then click foul play. So yeah, and again, if it is encore.
+6 252 Atk Wellspring Mask Tera Water Ogerpon-Wellspring-Tera Ivy Cudgel vs. 248 HP / 244+ Def Tera Dragon Mandibuzz: 345-407 (81.5 - 96.2%) -- guaranteed 2HKO
Lmao, not even an ohko.
+6 252 Atk Wellspring Mask Tera Water Ogerpon-Wellspring-Tera Ivy Cudgel vs. 252 HP / 252+ Def Tera Dragon Corviknight: 344-405 (86 - 101.2%) -- 12.5% chance to OHKO
Again, no ohko so they have a chance. Skarm has much more bulk and can whirlwind it out if the stall team has it.
252 Atk Wellspring Mask Ogerpon-Wellspring Play Rough vs. 248 HP / 0 Def Hydrapple: 206-244 (49.6 - 58.7%) -- 98.8% chance to 2HKO
Hydrapple doesn't need heavy investment to beat waterpon.
The last three are for stall specifically, which is what waterpon is best against. So even its best matchup has ways to beat it.
I do agree that the metagame is way too fast paced and something needs to go, if that's waterpon than that's fine by me. I won't cry about it as that's one less thing to account for in the teambuilder. Heck, if I had the choice I would vote ban just because something needs to go. Do I think waterpon is the first choice? No, not by a long shot. I have played all kinds of playstyles, from HO to BO to Balance to Stall and even semi-stall. Heck, when I made a dachsbun stall team, I slapped on tera grass to it so it could counter e-quakes from gouging, but it also helped against waterpon. I was trash at building stall teams, but I didn't go out of my way to account for waterpon.
Waterpon is also really weak to chip, it is so hazard weak which is not good in this hazard centric game. You should be accounting for waterpon on slower teams, but most teams 1. Have a faster mon and 2. Have natural ways to counter it, like with dragapult or roaring moon. The only teamstyle that doesn't have this is stall, which can slap on a tera dragon and a resist to make it work.
TLDR, I'm not saying waterpon isn't scary, it is, but it has crucial weaknesses. That is what makes it not turbo broken to me and only, "okay, this thing might be broken, though probably not".