The last part is happening. A lot of people switched to gogs/gitea/forgejo instances like codeberg when GH pulled a copilot on them. Lemmy went from being an obscure platform to a good one with lots of new users, better codebase and loads of clients when Reddit screwed its users. Mastodon was already healthy, but ballooned in size when twitter was trashed by Musk. YouTube is the only platform standing without a viable alternative, but people are trying after their adblock shenanigans.

Are the big proprietary platforms dead yet? No. Did they lose the audience - only a little bit. But it has made the alternative open platforms healthy and stronger. We are no longer in a condition where big platforms can just screw their users knowing that nothing will happen to them. Each transgression will cause more and more people to migrate. That's a good thing.