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.

It cannot be the worst when it's not one.