This is an interesting thought, I'm pretty sure it'll happen, but when?
I think Microsoft is slowly but surely losing. They keep looking to their next Windows release to be as big a hit as Windows 95, and never achieve that goal. That along with their failures in security (though they have improved), and hefty price tags, and the fact that non-MS OSs are getting better and better allt he time, I think that within a matter of years, maybe a decade or two, MS will fall out of being majority. More and more people are looking to Open-Source.