I don't think Windows will be open-sourced precisely because it's not as important as it used to be. It'd be a ton of work to root out vendor code incompatible with OSS licensing, remove internal dependencies etc. That's not worth it unless we have big plans for Windows to stay relevant, which I have no knowledge of but suspect that we don't.
Probably we'll see the most relevant pieces be opened up, like the driver model awhile back.