However Whatsapp/signal show how e2e can be done in a user-compatible way. By default it simply exchanges keys and shows a warning when key is changed and those who need/want can verify identity.
Missing there of course openness.
So the rest are actually OK with Whatsapp/Signal having the opportunity to see their messages? I would submit that most are not even aware of the issue...
The identity thing is basically the usability issue for E2EE messaging. If you don't solve that then you have not actually increased usability in a meaningful way. The PGP community understood this and did things like organize key signing parties. When is the last time anyone did anything like that for any popular E2EE capable instant messenger?