I think we have that already. It’s called ‘Haveno’.

What you describe as a possible solution is already implemented since decades. Most email clients give you the option to set and use a gpg keys or s/mime keys and will encrypt the contents automatically for you before sending it out, provided you have the receivers public key on your key ring, of course. But even that is too much work/too inconvenient for people to actually use it. Only massive pressure will eventually force people to use what we already got. Sad, but that’s the world we live in.