let's address the possibility that like mastodon/matrix 99% of ppl will flock to the biggest handful of servers
That's just human nature. There might be things to do, but they'll only have a small effect.
What is the real value of decentralization given that? Outside of like political unrest.
The ability to easily switch. A protocol to automatically migrate an account could he nice.
And what role do small servers really do in that landscape?
They keep the big servers honest.