"Mainstream adoption" is a very poor metric for success. Indeed - it feels it may be an anti-metric - the more mainstream mastodon gets, the less it is a refuge from that very mainstream.
User satisfaction is a much better metric. Lack of aggressive corporate presence. Fewer assholes and nazis and "influencers"- we left them behind for a reason.
Growth pursued for its own sake is the very thing that poisoned the other platforms. Let's not make the same mistake.
If your feed has interesting posts from good people and is *already* more than you can easily read in a day - it ain't broke, don't fix it. And if you don't have enough to read, ask around. We're happy to show you good folks to follow who are already here, and have been for a while now. There is already far more good content than anyone can consume with just a bit of self-curation - no need for more "mainstream", thank you anyway.