I could sort servers on the joinmastodon.org list by proximity to 150 active users (Dunbar's number), I don't know if that would be an improvement. The working set is still 60 top active ones in each category.
For example, sorting the "I am ..." set like that puts writing.exchange, lbgt.io, and monsterpit.net on top, and mastodon.social on the very bottom
On the other hand I do not see a compelling argument for why exactly Dunbar's number (number of personal connections you can maintain in your life) should be used. I don't think most people go into social media expecting to know every single person on their server, and having a headroom of people you don't know might even be a positive thing.
In any case I have absolutely zero proof that people at large use joinmastodon.org to sign up anywhere, that is to say, the reason a lot of people sign up on mastodon.social is because of high search ranking due to its age and a lot of links from the press. So all these potential changes might be absolutely meaningless.
I've also seen people say "their top server is only 200k users" as a negative when checking what Mastodon is, so consider that as well.
Maybe a stupid question but why don't you simply close registrations on mastodon.social?
@Gargron @roipoussiere I guess the question is do you want to become that you wanted to replace quickly or become that which you wanted to be but at your own pace? If we adopt the success criteria of the current mainstream, we will end up reproducing the current mainstream. If you want to control what a thing will become, make sure you’re the one writing the success criteria.
This is my personal Mastodon.