I'm not him, but I'm in the same situation: I currently use Gmail almost exclusively through IMAP, and I'd jump ship instantaneously if they turned it off.
Why haven't I left already? Because it works fine and does everything I need.
Why would I leave? The way you ask, you seem to think it's obvious, but I have no idea.
Yeah if IMAP access goes on Gmail that would be it for me.
I'd probably bounce back to hosting my own IMAP and using some utility to download my Gmail to it with Push or whatever protocol (HTTP?) that Gmail uses for push.
It would be nice to have Sieve scripts again to filter my mail (Gmail's filtering leaves much to be desired).
Because it's a hassle and it's easier to keep using it so far, and the alternatives aren't as nice in other ways. It's going to be a bigger pain to do it once I'm forced to, but…
(1) I don't have a server where I can run my own mail infrastructure -- I don't think my ISP would be happy with me running a mail server off my cable modem). And
(2) I don't trust Outlook.com not to silently discard mail from randomly selected senders (as I believe Hotmail was known to do). Gmail is reliable.
But otherwise, I'm ready. I've already stopped using the Gmail web interface (retaliation for killing Google Reader). And once I'm no longer tied to Gmail anymore, I can switch to a better IMAP client than Thunderbird.
Have you tried Fastmail? I switched to them a while ago and haven't looked back. Their web UI isn't as nice, but they have an honest business model with a much better privacy policy and migrating is pretty straightforward.
I like their web UI better than GMail. It is cleaner than GMail and much faster most of the time. It's also worth noting that they now have a beta version of a calendar (with CalDAV) support:
