And it's the best widely available, accessible, battle hardened, omnipresent messasing system.
I disagree and think rather that people have a parasocial relationship with it, like they do with IRC.
Incorrect - that would be WhatsApp.
What do you think of a system like Delta Chat built on top?
Trying to build a secure system on top of email is a waste of time and energy. Even if you succeeded, it would only be by compromising all the things that make email useful.