Already exists, already has a ton of traction, decentralized.
According to this, m.s has ~130k users which is tiny for a 6yo platform. And despite the renewed interest, some of that usage is people logging back into (but not really using) old accounts.
https://mashable.com/article/mastodon-twitter-alternative-el...
Platform demand is sticky rather than highly elastic because there are significant switching costs, some technical but most around rebuilding relationship trees. This is the biggest headache faced by communities who are subjected to deplatforming unless they have well-established lifeboat drills (a face of life on smaller chans and forums in the days when DDOS attacks were easier and accepted as a form of inter-community rivalry).
I don't see a mass exodus from Twitter any time soon, because the telegraphic nature of tweets and very strong network effects make it the idea outlet for people who like to complain.
