I'm less worried about Lemmy becoming mainstream, and more worried about if it's good enough for me. Right now, it seems more than good enough, and I love the fact that it's not relying on corporate backing or ad revenue.
Mastodon seems like it's approaching an inflection point, especially with the upcoming arrival of Threads. It sounds like Threads won't support ActivityPub on day one, but with that support presumably arriving in the near future, I think a lot of what's happening on the fediverse could be legitimized. I just hope Facebook doesn't do the same thing they did with XMPP ten years ago.