This is all-or-nothing thinking that's endemic to Silicon Valley's subsidies-as-a-service business model.

Mastodon does not have to monopolize microblogging the way Twitter has, because it isn't competing with Twitter's cursed ad-based revenue model. Similarly, the tiny niches that do migrate are likely more valuable to have than the hordes of Twitter users who never post anything, and just juice the algorithm by liking and retweeting.