Comment by AmericanChopper

The implementation details have some UX benefits, but all they do is kick the can down the road, not solve the problem. You need a secure channel to authenticate the keys (or “safety numbers”, or whatever you want to call them). This can only practically be done face-to-face (or by getting somebody you trust to do it face to face - to act as if they were a CA). You need to do this prior to first communication, and additionally every time somebody loses their key material.

Some people will be motivated enough to do this, most won’t, and this absolutely can’t scale.

All known PKI systems are either impractical, or require a level of trust that undermines the system entirely. You can say your threat model doesn’t require that much security, but in that case it probably doesn’t require a PKI either.