Until things like Thunderbird support it, its adoption is unfortunately going to remain at the level it is currently at.

Which sucks as it's a massive chicken/egg problem... more clients and hosts need to support it.

Of course, from what I understand is there are JMAP/IMAP gateway options, that should ease availability... assuming these things actually gain any traction ever.

Well email moves slowly for one, and adding another protocol is usually baked into a client and servers approach (imap, pop, smtp, etc) so its not impossible

I'm guessing thunderbird will be the first mainstream gui mua to support jmap.