Okay, I'm throwing my hat in the ring about Terastalizing, because a lot of the following pro-ban arguments being thrown out don't work in a vacuum due to the following reasons or comparisons:
"Pokemon become unwallable" = Manaphy
"Complete surprise factor" = Lure sets
"Gives them nuke options" = Z-Moves
"Too little investment for too high reward" = Mega Evolution
"Changes matchups" = Mega Evolution again, and even if it's a broader scale they'll still have blanket checks
"Overcentralization/steals games away with little skill" = Compared to Dynamax, not even close
"x becomes broken because of it" = Well maybe you should deal with x instead, because odds are it'll still be a problem even if you hit Terastalizing
"There's no counterplay" = Yes there is, because YOU can do the same thing as they can with just as little trade-off as they have
"Every team needs Terastalizing" = No they don't, because it's just a tool that makes matchups or already-good sets better, you don't NEED to always use it to keep up
"Makes every Pokemon become uncounterable because they could be anything" = Oh please, go play Balanced Hackmons, a popular OM where virtually everything is technically uncounterable by the same definition, and watch how pointless this claim becomes
The irony is that Terastalization is nothing new, because we've seen all of this before, and we've already accepted all of it.
Let me give it to you straight: Terastalizing is a symmetrical battle mechanic that makes Pokemon which are already good better. It doesn't centralize anything, it doesn't invalidate any playstyle, and if anything it gives players more tools to keep things in check than create newfound problems.
Personally, there is not enough grounds to warrant banning Terastalizing at all; this all seems more like a kneejerk reaction to change than an actual problem. If you want to reasonably restrict Terastalizing, such as a team preview if VGC will be doing it, then that's fair game. However, can we all agree that we shouldn't heavily limit or ban a new mechanic that does things that have never been a problem before?
"Manaphy" - Manaphy is a good Pokemon, but it requires setup to be truly unwallable and even then struggles to find moveslots for the right coverage moves. Besides, its middling speed meant it was very much possible to counterplay with faster offensive Pokémon.
"Lure sets" - Lure sets require more opportunity cost than Terastalization. You argue later on that it's a tool that makes matchups better instead of it being needed, while lures sacrifice other potential options a Pokemon could use. It qualifies as surprise factor but weakens Pokémon in matchups where they don't need it.
"Z-Moves" - Z-Moves could only be used by Pokemon with items. Terastalization can happen at any time, and isn't just a nuke but potential defensive counterplay to help you defend against threats you really shouldn't or set up on something that should be impossible.
"Mega Evolution/Mega Evolution again, and even if it's a broader scale they'll still have blanket checks" - Mega Evolutions are a unique case. Unlike the next three generational gimmicks, Mega Pokemon are unique to each and every Pokémon that has it, and therefore can be balanced around the meta. It's not like you could slap Mega Mawile on a team and expect it to win every game. Megas are undoubtedly very good, but because of being fundamental changes to Pokémon similar to having new ones altogether, they can be balanced in the same way regular Pokémon can, unlike the generational gimmicks following it. It also requires an item, so there's not a complete loss. On your point of Mega Evolutions changing matchups, many of the Pokémon stay the same type or are obviously mega. Seeing Lopunny in Gen 7 OU should immediately tip off that the Lopunny will Mega Evolve, while Pokemon like Garchomp, whose regular forms and Mega forms were both viable, stay the same type regardless and have very similar counterplay. Terastalization is not specific, and fundamentally the same for every Pokémon instead of different stat changes per different mon (although it definitely does not benefit all Pokémon to the same degree), along with being usable by every Pokémon, so balancing Terastalization is much different from balancing Mega Pokémon.
"Compared to Dynamax, not even close" - Sure, it's not as overcentralizing as Dynamax, but that itself was very overpowered, so it doesn't mean that Terastalization being less overcentralizing means it is not at all.
"Well maybe you should deal with x instead, because odds are it'll still be a problem even if you hit Terastalizing" - Terastalization can make bad Pokémon good and good Pokémon broken. I think that if nothing is done about Terastalization, we'll end up going down the line of broken Terastalization mons until more of the healthy metagame is gone thanks to inaction on Terastalization than not.
"No they don't, because it's just a tool that makes matchups or already-good sets better, you don't NEED to always use it to keep up" - You're make a point that there is counterplay, but Terastalization completely changes the fundamental core of many Pokémon, especially defensive Pokémon, and a Terastalization can easily result in a pivotal team member being lost at any time.
"Yes there is, because YOU can do the same thing as they can with just as little trade-off as they have" - It technically isn't necessary to Terastalize to win a game or even gain the upper hand, but that's not the problem. The problem is what I said before, it's beatable and not necessary, but Terastalization turns the tables on matchups that should be losses because of a failure in strategy earlier in the game or teambuilding.
"Oh please, go play Balanced Hackmons, a popular OM where virtually everything is technically uncounterable by the same definition, and watch how pointless this claim becomes" - Balanced Hackmons is in a power level much greater than OU, with an extremely wide range of possibilities. I don't think it's fair to compare the two at all.
And you aren't denying that all of this is offered at once. Sure, you can accept Rampardos's ridiculous 165 Attack stat because in every other department it struggles, which isn't the case with Terastalization. Of course, this is not to say you can't make arguments to say Terastalization isn't broken, but I don't think your points really work. Often times, especially in your earlier arguments, the way you structured your arguments feel like finding a specific measure to counter a very broad subject. I really like Terastalization, and would prefer if it didn't get banned, but it absolutely is worth considering a ban from the Overused tier, and your post hasn't changed my mind at all.