That’s literally what we did in gen 7. There are several Pokémon that got banned because a specific Z move set pushed them over the edge (Naganadel I feel is a really good example of this). I can think of 3, maybe 4 mons right now that are broken because of Tera, which honestly isn’t that bad all things considered. Complaining that Pokémon got too strong because of a new tool they got is silly, it happens every generation when new move tutors get added mid generation, which has been happening since at least generation 4. Greninja was very strong but still OU until it got gunk shot, Dragapult got banned in NatDex because of ghostium Z, etc.
I would agree with you if there were 6 or more things obviously pushed over the edge exclusively by terastallization, but currently the meta is full of things contributing to the Pokémon that are on the radar, it’s a bit of a stretch to pin Tera as the obvious reason things are broken right now, especially with the nonsense of shed tail and revival blessing throwing everything out of wack
Z-moves and tera aren't comparable.
Z-moves made you give up an item slot, and was basically a consumable (more comparable to gen 5 gems). Tera is a permanent buff that changes your typing alongside its nuke.
Nothing particularly was overwhelming because of Z-moves that I can think of, as the damage was comparable to running an actual item like specs or band in some instances.
If you look through the list of gen 7 ubers, you'll see:</strong></p><p><strong></strong></p><p><strong>Zygarde: was a fat threat not a Z-mover
Naganadel: Z-moves might've pushed it but overall it just needed 1 kill and beast boost broke it, gen 8 banned it too without any gimmicks.
Pheromosa: Really fucking fast with no checks due to its offenses and speed tier, even lando couldn't stomache ice beam with its mixed offenses.
Aegislash: This one Ghostium-Z was scary on
Landorus-i: just repeats what it did the past gens, too strong with sheer force with or without z-moves.
The rest: Megas that can't hold z-crystals, big legends that need no explaining, and blaziken
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To give an example of how the damage compares to tera:
252 SpA Volcarona Inferno Overdrive (185 BP) vs. 0 HP / 0 SpD Mew: 310-366 (90.9 - 107.3%) -- 43.8% chance to OHKO
Tera Fire 252 SpA Volcarona Fire Blast vs. 0 HP / 0 SpD Mew: 277-327 (81.2 - 95.8%) -- guaranteed 2HKO
Aprox a 9% decrease, however tera also means volc never has to give up HDB to deal very close damage, removes its bug typing, and can fire off more powerful fire blast throughout the game rather than once to power through something.
But usually you'd see this instead:
252 SpA Volcarona Bloom Doom (140 BP) vs. 0 HP / 0 SpD Mew: 157-185 (46 - 54.2%) -- 53.1% chance to 2HKO
Tera Grass 252 SpA Volcarona Energy Ball vs. 0 HP / 0 SpD Mew: 228-268 (66.8 - 78.5%) -- guaranteed 2HKO
Which instead is a 20-25% increase in damage, removes volc's weaknesses, and sticks.
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I would agree with you if there were 6 or more things obviously pushed over the edge exclusively by terastallization,
To give a list of things specifically broken by tera
Espathra: Turning fairy removes most of its priority move counterplay given only scizor is a viable OU bullet puncher right now, while giving its dazzling gleam STAB for more oomph against its dark type checks.
Roaring moon: Stab Acro and removes its fairy, dragon, bug, and fighting weakness
PissedOffApe: tera water lets it beat would be checks
Dragonite: STAB extremespeed, speaks for itself.
Chien-pao: defense teras make it harder to check/revenge kill, while tera fighting or its stabs make it muscle through teams even more than it already does.
Chi-yu: same as above, mostly just STABs though.
Dragapult: Gets stronger with tera ghost that nothing can really stomach.
Volcarona, Iron Moth, Kingambit, Iron Valiant: All held back simply because the above are still around, otherwise they could easily become broken through tera without the above keeping them in check.