The fact that JMAP has ~zero adoption outside Fastmail itself contradicts the "much needed" part of this title.

The problem is that email has been taken over by a handful of providers, all of which have "growth & engagement" as their business model and would rather not implement any open protocol at all.

IMAP remains supported for backwards-compatibility, but I'm sure its shortcomings are seen as a feature to push users towards the official clients so there is no incentive to implement another open protocol to address them.

Case in point: MS making it more and more annoying to use IMAP.

Hell, now you need to create oauth2 app just to login from non-approved client, as they recently forced OAUTH2 auth on IMAP clients. Sure thunderbird works but most lesser known clients require a lot of fuckery to make work (and possibly admin permissions for domain, not sure).

"Just works" I run it with mbsync at the command line.

You do need a client id and some interaction with your O365 admin.