Lemmy

let's address the possibility that like mastodon/matrix 99% of ppl will flock to the biggest handful of servers

This doesn't appear to be happening in a serious way. There are 7 servers on join-lemmy.org with more than 100 active users, which puts them within 10x of lemmy.ml. It's not clear to me that this is a major problem.

But even if users to substantially centralize, these things make Lemmy categorically different from centralized social media:

• Even just two instances that interoperate and are run by different people/groups is MUCH more robust against bad admin behavior than 1 instance with no interoperability. If either set of admins starts doing something extremely unpopular, there's very little barrier to folks migrating to the other instance. Doing so doesn't even cut them off from communities on the other instance... Though if the problems are serious whole communities can move as easily as large groups of people can agree to do anything together.
• The ability to stand up new instances makes Lemmy MUCH more robust against the admins of the major instances colluding to something extremely unpopular. It's not EASY to stand up a new instance to serve a large user count, but it's possible, which is more than can be said of reddit/Facebook.

These two properties act as incentive to "keep admins honest", and as a powerful escape hatch if they behave badly. They have a cost in that it's easier for communities to splinter for stupid reasons, and for people to "take their toys and go home" over small disagreements. But they make the ecosystem as a whole much harder to hijack. If people aggregate on big instances, it's because those instances are run well and people like being there. If those instances stop being run well, the ability for anyone to stand up alternative instances that interoperate enables people to leave badly run instances with a much lower opportunity cost than leaving a centralized social media service like reddit, Twitter, or Facebook.