So you’re fully aware that Tera restricts teambuilding and creativity to a significant degree but you don’t want to do anything about it because the dex is too small? I’m a little confused at what you’re getting at here. The dex was small at the beginning of last gen too and Dynamax was still broken.
I don't think tera restricts teambuilding right now, actually. With regards to the single biggest threat that does not belong in this tier whatsoever (Chi-Yu), it lets you improvise some really bizarre counters to it.
Chi-Yu is really only disgusting and capable of destroying something every single time it hits the field if it's choiced, and especially vs Stall where it's game ending. What people are doing more and more is saving their tera and using that to soak a hit and OHKO it. I've seen fairy Dondozo, ground/dark Clodsire, fairy Blissey, fairy Garganacl, probably a few others that aren't coming to mind. Right now ironically enough tera is stall's only option to dealing with Chi-Yu.
Yeah again 100% it needs to be banned but again, tera isn't restricting teambuilding there. The only other theoretical counter I can think of with tera gone is Thunder Wave Chansey/Blissey. Now at least Stall has 4+ options.
Looking to a more non-tera dependent example and using a setup sweeper, I ran a more offensive team that had issues fighting DD Dragapult. I could have just slapped Dondozo on my team and called it a day, but what I did instead was take one of my dragon types and have it go tera Fairy. Now it perfectly absorbs Dragon Darts while living whatever other option it would have thrown, and I respond with a Draco Meteor.
Now granted that is prediction dependent and he could see that coming and go Fairy himself, but at least then it's fairy and tera is off the table, and now something like a Scizor Bullet Punch OHKOs it. Tera allows me to run a bit scant in a few areas and rely on type switching to compensate. I'd argue that rewards proper prediction and teambuilding, and while it does make a few offensive threats stronger, it gives you defensive answers to that same offensive threat.
It's almost like a samurai movie where the first to Tera loses.
Most teams looking “pretty much the same” isn’t exactly the sign of a healthy tier, it’s the sign of one that’s either overcentralized around a handful of broken things or stagnated to the point where people can’t be bothered to try new things anymore. (Basically, either the very beginning or the very ending of a gen.)
Being effectively forced to run one Pokémon on every team is unreasonable, we agree on that (and hopefully so does everyone). I also believe that being effectively forced to run at least one of three Pokémon on every team of a certain archetype isn’t healthy. The number of Pokémon holding together the tattered skeleton of what used to be stall and bulky balance can be counted on one hand.
I think even with tera gone we have this exact same problem. Landorus was a blanket physical wall alongside being a defogger and a stealth rock setter (not that he was a stall staple but he certainly was for balance/BO). Now every team that wants a physical wall looks elsewhere to Dondozo or physically defensive Skeledirge, and then you still want some Stealth Rocks and that's Garganacl/Blissey/Clodsire, and then there are all of like two pokemon that learn defog so you're throwing on Corviknight or maybe going more offensive with Cyclizar, one of the Donphans, or maybe Maushold.
That's just for Landorus. There are plenty of others that everyone wishes they had right now that we're simply without - including a bulky Fairy type that's actually fairy and doesn't need to tera to pretend it is.
When inevitably we get a suspect ladder without tera, if it is still pre-home, I think you'll see things are basically the same. Our options are so limited not only in Pokemon but in what their movepools have become that it's like gen 2/3 again where every team within a given archetype is largely the same.