Mastodon is a bit like email. Not great but it works right now and is reasonably easy to get into and integrate with. The feddiverse is bigger than Mastodon. But it's mainly Mastodon. And Threads soon apparently. Millions of people daily active on it. Not quite critical mass yet but closer than most other things have gotten since Twitter started imploding. Good enough?
IMHO mastodon with signed content and per user key pairs would make a lot of sense and it should not be that hard to add it. Why is this not a thing? Just like using pgp makes a lot of sense for email. Except that never happened either. Already when I started using email in the mid nineties, some people were insisting on using pgp. That's thirty years ago and it was only some people. It remains very uncommon for people to use that. Key management is an obstacle. Techies get it. Everybody else doesn't.