Windows will go open source when pigs fly
This will happen when pigs fly, or John McCain wins Dancing with the Stars. (Picture from Delphi Consulting.)
Open source is not a date. It's a commitment, like marriage. History has shown you can't be a little bit open source. You're either in or you're not.
Open source requires that you change your business model around entirely. The community becomes a big part of your development effort, but it may not want to go where you do, and you have to accept that.
Open source also puts a limit on your potential revenue. Not everyone will pay, in any way. You either accept that or walk away. Try explaining that to the investment bankers.
Microsoft's bigger problem is that Windows has become unwieldy and too-integrated. If they opened the code to inspection my guess is the laughter wouldn't stop for six months.
Besides, can you imagine how many lawyers will be crawling over that code base looking for open source snippets to sue over? He who lives by the lawyer will die by the lawyer.
Most important I don't think the proprietary model is obsolete yet. It dominates in games, in interfaces, and in security for good reasons. As Darrell Royal said, you dance with the one that brung ya.
And finally who would we have to kick around if Microsoft went open source?
I'd have to rename this blog source.