An open-source protocol won't prevent a platform from accepting content only from approved sources and serving it only to approved clients.

Facebook could start using on open protocol today, then block anyone not signed in through facebook and the result is nothing at all would change.

Maybe you're thinking platforms will somehow also be required to accept and share content from anyone? If so, all platforms become like 8chan (or worse). If not, there's someone, somewhere deciding what can and can't be shared on each platform (like there is now).

I think the fundamental confusion here is the idea that some kind of protocols vs. platform dynamic even exists.

Social media platforms are trying to achieve a captivating user experience built off our social nature to monetize our attention. Protocols are used, but they are deeply buried implementation details.