This applies to much of the XMPP discussion above, too. Also worth noting that GnuPG isn't even what you send the emails with. And that PGP on mobile is almost unheard of. I doubt I could explain how to send an PGP encrypted email (for example, to a doctor/lawyer/accountant) inside of 2 minutes.

Strong intrinsic motivation is the only reason people use PGP. It took me ~3 months to get a critical mass of 15 or so people on to Signal as first port of call.