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).
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.