It would be interesting to see what might happen to Linux and FreeBSD if this were to happen. With Windows joining the party, I could see a legion of open-source coders writing and adapting their code for a UNIX-based Windows, and vice-versa. Hardware vendors would find it much simpler to write drivers that would support Mac, Linux, and Windows.
Microsoft would benefit greatly from enhanced security and a smaller footprint. Developers and vendors would benefit greatly from a simplified codebase, and users would benefit from a wider range of available software, and obviously the enhanced security.
The Internet would obviously benefit greatly from all of these things. In time, even the monster Windows-driven botnets would die, and spam would be reduced.
I ended my last discussion of this topic with a Henry Spencer quote: "Those who don't understand UNIX are condemned to reinvent it, poorly." It's as true now as it was then. It'll be true for a long, long time.
So take the challenge, Microsoft. Innovate. Adapt. Cut your losses. Start replacing your foundation, lest the whole structure collapse.