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.

Google has never had a standard IMAP implementation for Gmail.

I still don't see the trouble. I use imap for work email, because our corporate web mail looks like yahoo circa 2004, but gmail I use chrome, a far better user experience.

With Gmail, you're limited to seeing mail from one account at a time. With Mail, it coalesces email from all my accounts into one list.

not at all. gmail can pull from many imap or pop sources.

At $work, we use Google Apps. What happens to people in my situation, who were using Mail.app instead of the web interface?

you use the web app, as previously mentioned

What about in-app mailing support? And notifications? Gmail's web UI seems like a suboptimal way of doing email on a desktop OS.

Some people like encryption.

Or Postbox, which also still works just fine under Mavericks.