Ok, you're crazy. The GPL licensing issues alone would be a nightmare for them, and possibly unsolvable from their business perspective.
I cannot imagine a BSD-based MS Windows but it would be cool!
This would probably solve slow filesystem I/O (which is one of the major pain points for me when working under Windows, especially if you have a large number of small files).
However, it's important for Windows to keep backward compatibility for applications using old APIs. I think that's a major selling point vs. other platforms.