Could a fuse filesystem do the trick for maildir emulation? Assuming that'd be beneficial to a full maildir, but I guess it'd only be so at large sizes.

Couldn't à hypothetical fuse fs emulate any file based store for email?

The strong suit of JMAP (among other things) is getting access to specific properties of a server side parsed email. It will do the MIME parsing for you; separate attachments and text bodies from each other and for example allows a client to fetch only Subject and senders instead of the entire email. By brainlessly dumping all that into a Maildir you'd be giving up a lot of the benefits of JMAP. I'm a big fan of JMAP but I don’t see it replacing IMAP for (desktop) clients that will keep a fully synced Maildir locally.

Nobody said anything about doing brainless things. There's a lot of sound and fury from Fastmail about JMAP, but the only client is ... Fastmail's web interface.

I don't care what JMAP wants to do for me as long as it's not actually available to me. IMAP supports downloading headers too -- that doesn't mean everyone uses IMAP 24/7. The IMAP + local synced maildir use case is extremely common and there's no reason JMAP can't do it better.

I merely suggest that starting by making small shims for interoperability would encourage protocol adoption so that when clients arise they'll be in a position to be successful out of the gate.