When I transitioned from Gmail to FastMail, I had to get used to using folders again instead of labels. While it was painful, I am glad I am no longer using labels, and I have no intention to start using them again. I vastly prefer being able to rely on the basic email client functionality in any given mail app, and using goofy nonstandard parts like snooze and labels makes that difficult. I appreciate that with JMAP…
Is there any reason for why IMAP keywords cannot be used for labels by e.g. having some prefix for the keyword which indicates that it should be interpreted as a label? Client support is of course needed but to me it seems like the IMAP protocol already supports labels. Maybe IMAP lacks the ability to enumerate all labels?
So the issue is that any other solution would work improperly for Gmail. Client support is a huge part of the problem: If it's outside the standard, clients aren't going to handle it the same way.
JMAP, of course, fixes this.