Mastodon positions itself as a place where Twitter expats can go, but not directly as a Twitter competitor. This is something a lot of people aren’t catching. Mastodon isn’t a company looking to monetize a social network. The community appreciates user growth because it means a more robust network, but the primary goal for the community (to the extent that there is a shared goal at all) is simply to build a social media space free of the incentives and by-products that come with ad-tech, algorithms, and proprietary social graphs for people who want that type of social media experience. It doesn’t need to outgrow Twitter to realize that goal. It’s not playing the same game as Twitter and many other social media companies.
Could the UX be improved? Yep, and work is being done on this. But there’s no urgency to massively overhaul the way Mastodon works. Again, that’s not the game the Fediverse community is playing. All the network needs is a critical mass of engaged users, and enough brain drain has happened from Twitter to Mastodon that it has probably achieved this critical mass.