I think there should be two parallel networks:

• anonymous one run by private corporations or volunteers with heavy moderation and the company taking full responsibilityp for anything posted there. pretty much what EU is trying to establish. You want to make money of anonymous posts? You take financial responsibility for making sure there's nothing illegal on the platform. You could still have right leaning sites but with no misinformation or harassment. Fedivers instances that notoriously post illegal content would get the same treatment as neo-nazi sites.

• publicly owned platforms (like mastodon instances) available for everyone for free but with no anonymity: you want to make something public, complain about something or simply interact with normal people you can always go there. Less moderation would be needed so it would be cheaper to run. Users will be responsible for the things they post, not the platform.

This way if Twitter is unable to moderate their content you block it and people would have public instances as an alternative. We would see if running a platform like that with proper moderation is still profitable. If not they would start charging people or shut down. I don't think losing sites like Facebook or Twitter would a big problem if we had public alternatives.