They complain about complexity, then add another two layers of complexity in their own protocol?
I think POP3 is the simplest standard, and have also written a basic IMAP client. Parsing IMAP isn't as easy as a binary protocol, but it's definitely not at the level of HTTP JSON bloat that seems to have infected all "modern" protocol designers. I can use POP3 reasonably easily from a netcat (and have done so many times in the past), and IMAP is a little harder but doable. I don't expect that to be doable for JMAP which is text-based like the other HTTP JSON bloatocols, but unlike the earlier text-based standards like SMTP POP3 IMAP IRC MSNP etc., it seems to have all the disadvantages of a text-based protocol but none of the advantages.
POP3 has no locking. It won't work with multiple clients, and that is awful. But I agree with your assessment. We need good utilities which make JSON human-readable, and which turn human-readable into JSON. Might as well not use JSON, except for internal traffic.
If only one client writes data or if you constantly do full sync (quite expensive on resources), yes. You can't even have different maps to sort content though. You don't follow states. POP3 just doesn't scale well in the end. If you're in 1995 and you get 3 emails a week and get really excited you got an email, then it works very well (I still hated the full syncs on dialup though). I used POP3 for several years in the 90s, even wrote my own client (a very simple protocol) Then end 90s, I used Pine and then Mutt directly in a shell on my ISP, with IMAP and Maildir; a relief! I've never used JMAP, since 1) I quit self-hosting email due to (justified, given spam) complexity of the stack and 2) I don't use Fastmail but Soverin.
They cited IMAP's complexity, high resource use,
JMAP is implemented using JSON APIs over HTTP
They complain about complexity, then add another two layers of complexity in their own protocol?
I think POP3 is the simplest standard, and have also written a basic IMAP client. Parsing IMAP isn't as easy as a binary protocol, but it's definitely not at the level of HTTP JSON bloat that seems to have infected all "modern" protocol designers. I can use POP3 reasonably easily from a netcat (and have done so many times in the past), and IMAP is a little harder but doable. I don't expect that to be doable for JMAP which is text-based like the other HTTP JSON bloatocols, but unlike the earlier text-based standards like SMTP POP3 IMAP IRC MSNP etc., it seems to have all the disadvantages of a text-based protocol but none of the advantages.