You just have to (encourage others to) register on an instance with less than, say, 2000 active users. I think that’s already taking care of most of the issue.
Yes and another thing that also doesn’t help is that Mastodon currently does not (yet) support migrating over previously posted content. So if you migrate from one server to another, your old profile does show “user migrated to X”, but I think people are a bit afraid to leave their old posts behind.