I don't agree with any of this mentality.
Players will adapt. The balance will adapt.
I am not some "Pokemon is the GOAT!!! Love the status quo!!!" person, I play tons of alternatives to Pokemon, such as other monster collection games. No, the game will not shit itself is Steel does not wall almost everything in the game. Because nothing could.
Nothing is truly """broken""" in competitive Pokemon beyond a metagame level unless it disrupts the mechanics of the game. Hell, if you had a type that is literally super effective against every type in the game, if you could Toxic it, get hazard damage on it, use moves to lower its damage output and items like Sash to combat it, the core mechanics of competitive Pokemon would still be at play.
Would variety be ass? Sure, but that doesn't make the game "broken". Metas like CHALK were not broken, they were fine because they were playable.
So from my perspective where even that would not break the game, the idea of "omg how will we fight a bad type if Steel doesn't resist it" is funny. If Psyspam is broken in VGC, they will adapt, as they have every time. Psychic being a top tier type wouldn't break the game, it'd just change the status quo of what is on top. The status quo balance is not good, or bad, it's just what exists. It's what you're used to.
If you want to see "Psychic Type but Steel doesn't resist it", just look at Ghost. While you use Psyspam as an example of brokenness, Ghosts are just Psychic Type but better. You can say Psychic has things such as Psychic Terrain, but any type change will be accompanied by a new entire game where you could nerf Psychic Terrain.
This isn't some impossible game design task. It's literally never been. You bring up 1000 Pokemon, but that isn't even true anymore. Even if it was, you say "it's not like everything is just OU, there is other tiers"
First of all, VGC is way easier to balance. Inherently. Because you always have the ability of double targeting a Pokemon making it less viable. Mega Rayquaza is still insane, but even if it can 1v1 almost every Pokemon by sheer speed and DPS, you have a second Pokemon that can attack it on the turn. That isn't to diminish that Mega Ray is pretty broken, but to say that it is inherently easier to play around threats in VGC.
Secondly, if you are implying lower tiers Smogon: who cares? They can just ban broken Pokemon and continue as normal. It'd literally be fine. In the status quo already, we acknowledge that Pokemon is an extremely busted game, with tons of things that should be banned.
So actually: Yes, there really isn't that many Pokemon to take account of.