The prices of "unlocked" phones would skyrocket. I could live with my Rezound and ICS for a good long while. I'm sure CM and some others would continue to support development. But it's such a very extreme remote possibility I'm not even going to bother.
If MS could actually get something right for a change I'd be interested to check out a WinMo experience. Exchange is still my primary need for a smartphone and even on Android it isn't as good as it could be without some hacking (exchange play nice with universal inboxes, at least as far as widgets go).