We should not be optimising Mastodon so it can handle more people per server. We should be optimising Mastodon so it incentivises more serves with fewer people.
(And if you take that line of thinking to its logical conclusion, you arrive at the idea behind the Small Web: https://ar.al/2020/08/07/what-is-the-small-web/)
@aral i've had thoughts about reworking the old concept of relays into a sort of overlay network -- joinmastodon could essentially work like an irc network in having servers that users connect to round-robin, but discovery could be handled at the central gateway service. that way if the central service goes down, you can still use your server, you just get a less complete view of trends and it becomes slightly harder to find profiles. then it should matter less where you sign up.