I'm going to assume you're serious and agree with you; I wouldn't be at all surprised if Microsoft did end up joining the Linux Foundation. I'll guess within the next 2 years.
The business unit that would open source Windows is Windows Development Group, who are mostly stuck in neutral. They can barely perform builds of Windows themselves, it's so huge and tangled and poorly organized. They can't change anything because they're crippled with decades of technical debt. Whether they want to or not (and I don't think that they really do want to) to try to open source that monstrosity is quite far away.
That said, I'm sure some people within Netscape said the same thing about Mozilla. So I'd love to be proven wrong.