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.