That's a bit harsh given all the problems SMTP and IMAP have. They are very dated and have some exotic "features". There is not a single mail client that gets everything right!
In other words, “they don’t sit atop the currently-fashionable implementation techniques that are the only thing webshits happen to know.”
Well, no shit, they were designed before HTML and HTTP, not just JavaScript, and there’s zero need to actually shove them into that mold because they already exist and are fine as they are in terms of protocol-level features. If you can’t deal with that maybe you shouldn’t be trying to do development at that level.
Meanwhile, all that energy spent reinventing the protocol layer to be maximally aesthetic to people for whom all the world is JavaScript running in a browser could have been used to actually improve the applications running over the protocol.
JMAP really solves many problems and allows you to do many things that are just not possible with IMAP. Also, IMAP has many ways to (badly) perform the same task, not all supported by all servers or all clients. MIME parsing is not easy to do and there is not always quality libraries available for your language... JMAP solves all of this.
