We both know that is not how this should work. If everybody joins the same instance

But this doesn't happen. Mastodon.social is far from 90%, and the former CEO made the choice of letting control go to an entity that is no-profit, I think it's the right choice.

• The casual user joins mastodon.social (default highlighted choice). He can enjoy the Fediverse and discover its complexity with experience, not by understanding everything in advance.
  • He can the move to another instance if he feels so, after undetstanding what this actually means
• The more advanced user probably already knows the instance thing, and maybe doesn't even use the offical app

I think it's the "basically" part in basically right back where we started that makes the difference. Even if 100% of Fediverse users were on a single instance, once that instance starts pulling Musk moves, the users can move somewhere else.

Only if that instance defederates from everyone else do we get the Twitter situation again. Or, since that's actually what happened, Truth Social, which I think is just a slightly modified Mastodon instance with federation disabled/deactivated.