Designing a new protocol is the easy 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.