Replace "huge Gmail user" with "huge email user that happens to use Gmail servers as the host" and you can begin to see the trouble.
It shouldn't matter which IMAP client you use. If there's trouble in communication between Mail.app and Gmail's IMAP servers, then one of the two parties messed up. If there's a regression in Apple's Mail.app, it is a big deal if you have an established workflow with any piece of software. Likewise if Google is making it harder to use your IMAP client of choice, it's a big deal. Users have established patterns and they're hard to break. Hell, just re-syncing my whole email archive into another client would take days (Google throttles batch downloads aggressively, and I don't always have internet connectivity when I need access to my email archive). Nevermind learning a new user interface.
When software regresses, users will complain.