I would be very surprised if MS succeeded in ditching Win32, but if it came to it, it would be very easy to make SWELL (and thus REAPER) target Polaris (in addition to Cocoa and X11)... but then you'd have to deal with all of your VST plug-ins, and suddenly Linux desktop w/ WINE would be looking pretty good to pretty much all serious Windows users now. And thus you see how big of a hill MS has to climb.