CoreCLR is now open source
I would be curious to see the effects of completely open-sourcing Windows.  Businesses would continue to use it, because it's Microsoft and they want enterprise support.  I think it would get even more love than it already does from the development community.  Piracy of Windows is already rampant, so they're not really in a worse position from that (plus I think that most people who can pay for Windows do so already).  Foreign governments who are concerned about NSA backdoors would have their fears allayed.  Is there any way it could seriously damage their business model?