I just believe, in today's atmosphere, synchronisation where encryption is a second-class citizen is a missed opportunity.
Fortunately, Fastmail has a blog post that goes more in depth on the subject: https://fastmail.blog/2016/12/10/why-we-dont-offer-pgp/
What you want is something on top of emails, like autocrypt (https://autocrypt.org/)
That doesn't begin to solve the real issue of how to reliably encrypt from sender to sender across two servers it just makes the process even more needlessly complex.
> Clients that want to or need to (for example those doing PGP in the client) can still fetch the RFC5322 if needed.
Not a case of JMAP saying to the client, "Hey, here's this blob. It might be encrypted." But a case of asking the client to use a message standard they've replaced where possible.
That's what I mean by second-class citizen.