What prevents you from designing a new protocol? I agree though with the monolithic one company app part.
But for that new protocol to be useful, it would need to:
(1) become an open standard which anyone could implement, and
(2) be built into every major platform.
For example, if I want to send a message to someone, let's say my doctor, I can't expect them to download some app or create an account on some new web site to be able to read it. E-mail doesn't require that, and any protocol that does would face huge obstacles to being adopted by the general population.
[1] https://www.ietf.org/blog/jmap/ [2] https://datatracker.ietf.org/wg/jmap/about/