I don't buy it. The issue isn't that Open Source would be disruptive to Microsoft's revenue stream, but rather that it's incompatible with their business model. It's not about licensing fees, but control.

Microsoft's business has been built about leveraging their exlusive access to APIs and data formats into other market areas. I think Microsoft may be willing to give away their OS for free (as in beer, not speech) before they would expose the proprietary interfaces of the system.