The fastest route to MUA support is compatibility with existing MDAs like procmail, maildrop, and the like. At the very least a JMAP client that populates a local Maildir would get you Thunderbird, Evolution, mutt, and more.
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.