But end-to-end encrypted email? It breaks everything. You need to get all the MUAs to support it (very few do either S/MIME or PGP). You'll break webmail--the most popular way to use email--without lots of major investment. And encrypted email breaks things like spam filtering or server-side filters catastrophically. Key discovery is also unsolved.

There was a time when I was on the everybody-should-use-encrypted-email train. But I've since grown up and realized that encrypted email fundamentally breaks email in ways that people are unprepared for, and people have already figured out how to route around the insecurity of email via other mechanisms.