Surely the problem with large servers is that they're difficult to moderate, difficult to grow a sense of community, and all the other problems that lead the OP to suggest that 5000 is a vague upper limit.
If you are going to go with bigger servers, what is Mastadon doing to support them (socially, not technically)?
I think really huge servers will need a different moderation system. It wouldn't be the responsibility of the Mastodon team to manage or even design that.
Also, mastodon.social has about half a million users, and they are having performance problems. I'm guessing if you want to scale that up by a factor of 10 or 100 you're going to need new infrastructure.
Surely the Mastodon team are very concerned with moderation? Given that everyone agrees it is vital for the success of the project (or any social network), why would they consider it outside their purview?
I meant that they wouldn't consider the difficulties of moderating a multi-million user instance. I believe they have enough to deal with when it comes to moderating their half-million user instance. Also I think a large number of those users have either stopped using it, or moved to different instances.
If you are going to go with bigger servers, what is Mastadon doing to support them (socially, not technically)?