People say things like that, and I wonder if I’ve just been living in a gilded tower of using Apple Mail with decent IMAP server implementations.
I’m also pretty familiar with the wire protocol and its implementation — it’s never struck me as particularly horrible.
A new protocol isn’t likely to solve the problem of poorly implemented clients and servers — e.g. Google doesn’t really care about good IMAP support, so they’re unlikely to care much about JMAP, either. They just want you to use their webapp.