Another reason is that webmail vendors of today typically deliver a subpar IMAP/POP experience. So combining a native client and webmail when on the go is rarely ideal.
JMAP is the first chance in a long time to get a decent experience that doesn't involve an expert and (even if you are willing to put in the time) tons of compromises.
And aside from all that we have an opportunity and reason to revive native clients that has gone stagnant (which is pretty much all of them).
Could you elaborate on this? I personally find using a mail client like Thunderbird to access my mail account far more responsive and seamless compared to using the webmail interface. If I don't have much bandwidth to work with, the local mail client using IMAP and SMTP will work while the webmail client will experience timeouts.
But syncing mail with IMAP isn't good enough and many features of webmail clients do not map to IMAP clients. There are also tons of caveats during configuration so tons of people, rightfully so, don't bother.
How would JMAP accomplish that, in a way that other protocols don't?