I agree with the other commenter that it sounds a bit like the Fediverse. It’s interesting to think about. I think part of what draws people to any messaging platform is continuity with the other services on the platform. The actual messaging experience can be duplicated or exceeded by anyone, like how RCS has made the humble text message more powerful and compatible than anyone at Apple could comprehend.
With this idea, would any messaging platform that became ultra successful be then required to allow other platforms to message their users? Which platforms are allowed? How is spam managed? What about special privacy features like what’s built in to Signal or Telegram? How do the platforms manage linking to content embedded in other parts of the platform (think Instagram posts/reels/messenger).
There are a lot of difficult issues to work out.
Vimeo is not for the same purpose. It’s more B2B. I read somewhere that after a certain threshold they start billing you for views.