What was Musk's biggest bugbear about 'old' Twitter? The moderation system. If Twitter goes under, the only practical alternative at the moment is Mastodon, and Mastodon can't replicate Twitter's moderation due to its architecture. So Musk gets what he wants anyway. He doesn't have much to lose in this scenario: Financially, he'd remain a very rich person. One can argue he'd be better off without Twitter anyway.
Lots of instances also screen signups. There's no requirement for them to accept everyone. The server that Marcan (M1 Linux) and Alyssa Rosenzweig (Linux GPU drivers) are on claim a total of 700 active users for example. You'll still get new content as your users can boost (retweet) from other servers, while they act as gatekeepers by not boosting others.
Tumblr announced they would join the Fediverse. That would be interesting. Of course nothing stops Twitter from joining either.