> For Facebook, Instagram, Twitter, each person having their own website where they post and that post being pushed to these platforms is also another way to force interoperability on them or be left behind.

There's an acronym for this: POSSE (Publish [on your] Own Site, Syndicate Elsewhere). Part of the IndieWeb movement, for those who want to explore this worthwhile idea further.

Sure, you can do that. But then the syndicated content usually ends up looking like low-effort slop and doesn't get much traction. Each publishing platform has it's own features, limitations, and cultural norms. If you want to have any impact then you can't just copy content around: you have to tailor the message to the medium.