That is why protocol fiddling is an essential part of it. Not enough, but without something better than IMAP might as well give up yesterday (as most already have done).
How's that? I don't see how messing with IMAP/POP/JMAP will make a difference there. Mail delivery, over SMTP, is what the Gmails and Outlooks of this world are so nasty about. They claim to protect their users from spam, but small mail servers get killed in the crossfire. Mail clients downloading mail over whatever protocol is a trivial problem by comparison, and they have zilch to do with SMTP abuse.
If 99.9% of all mail originate from gmail+outlook then they can (and will) pretty much ignore the rest.
The less dominant they are the more they are forced to be reasonable.
JMAP will help diversify the market as well as simplify the process of creating competing products. And has the potential to vastly increase demand for native clients and proper support from webmail vendors.
All of the above will work towards lessen the dominance of gmail etc.
It isn't anything by itself but I believe it is a requirement going forward.
Because IMAP doesn't cut it, so everyone has to create their own crap, which won't be as "good/easy/cheap" as gmail.
But you can't send mail to anyone using gmail, which is nearly everyone, unless you play by Google's rules. And even then they'll still put you in their spam folder, or just randomly blackhole you without any explanation or recourse.
JMAP vs IMAP vs POP is moot as far as I'm concerned. As long as my mail client continues to work, I'm good. If this gains any traction, I'm sure dovecot and the likes will add support for it (if they haven't already). But no amount of protocol fiddling can fix the fundamental issues of email centralization.