I've been categorized as a super pro-Tera poster before (probably understandably so), but over time my opinions on the mechanic have become less gung-ho, and in my opinion, more nuanced.
Simply put: I believe Terastilization to be a sidegrade to Pokemon, and not a positive or a negative. It changes the game in a way that will either be to someone still competitive, and probably more fun; or uncompetitively, unhealthy and degrading to your enjoyment over time.
I'd say that there is absolutely a mindset change that is required in a Tera metagame, and that is with long-term or short-term thinking. In The Olden Days of 8ish months ago, or literally now if you go and play old metas (that exists btw!), the common experienced practice is to load into a match, read what you need to do and what your endgame will be, and play to that endgame.
Now, with Tera, that endgame could be impossible from the get-go. Or very hard to pull off compared to another. It adds a different air to the game, where short-term strategy matters more. The endgame decision is much later in the game (at least compared to Turn 1, especially when half these games be sub 50 turns for real lol), and that can lead to, arguably lesser quality matches
if that is what you are looking for in competitive Pokemon.
If what you are looking for in competitive Pokemon is a game where you are playing to an endgame from the start, and must execute on that before the opponent then you will probably see Tera is unhealthy. It absolutely, 100% makes that a lot more foggy, and possibly even "less competitive"
if that is what you see as competitive Pokemon.
However, not everyone sees competitive Pokemon's value as that trait. I cannot speak for other people, but to me, competitive Pokemon is very enjoyable when I know what the context of the metagame is, and I am able to adapt throughout the match.
In short, I, and I presume some others as well, value short-term planning as well; and adaptation of a player. The ability to go in with some idea, but then have it dismantled, only to be able to come out on top is in my opinion also competitive. And this is
absolutely a thing in almost every metagame, it's just less important.
In other words, this is why I honestly really enjoy Gen 3 and 4 OU, where meta knowledge can help me figure out cores, but it's not really guaranteed until I play more. There is discovery in all metagames, but
I do not think it is valid to assume every metagame tests the exact same skillset to the same degree. I think every metagame tests every skillset to some degree, but different ones have different priorities.
long post so here is a picture to break it up take a break woooo :>>>>
I am also a Random Battles Enjoyer, I am deadass much better at it than regular OU, and while ironically Tera in my opinion is pretty ass in Random Battles (because it's unironically more swingy than Dynamax in the mode, as it generally does not bring much defensive value. If I Tera Steel my Pokemon to take some Outrages, their last could be a Fire-Type that my otherwise Steel-Type could beat. Not to say Gen 9 Random Battles is uncompetitive, I just don't really enjoy Tera in it.)
In short, I can see why someone would find Tera unhealthy or uncompetitive depending on why you think competitive Pokemon is, er, competitive in the first place. As a player that values adaptation above long-term game plans, I do not see Tera as problematic. I can see why others would, but I do not think that makes it "objectively" uncompetitive, or unhealthy.
On the topic of Pokemon themselves, in a very old post I talked about how a pro-Tera arguer simply has to accept that some things will be banned because of Tera. I still agree with that, but I will say most things "broken by Tera" are already pretty matchup-fishy Pokemon that basically win on the spot without set counters. Espathra isn't exactly fucked without Tera, it's just less able to be an uncompetitive force in the meta. I'd argue Volcarona has never been good for a metagame, and if it eventually gets banned here (which so far seems unlikely, alas :/ ) is just able to be more obviously good at doing what it's basically always done, just more blatantly so.
This was a bit of a rant, but basically; I sympathize with how it must feel to not like Tera, but I feel like pretending it's just "one side is facts and logic" and one is "it's fun!!!" is not healthy discussion, or valid. I think Tera shows what people prioritize in Pokemon, and that it isn't all the same thing.