I voted maybe, because it all depends on what kind of OS would it develop then. If Windows was a Free Operating System that would also mean that it would benefit from the network effect and peer review meaning that it could have potential to develop into an outstanding OS.
If Microsoft really cared about quality of their OS more than about their fat bottom line, they would serious consider freeing it up, but that's just not MS.
While I consider myself a GNU/Linux advocate I am more fundamentally a all Free Software supporter meaning that I don't really care so much about which OS is used as long as you have freedom with it. And that's still the biggest issue I have with Windows. Of course, lack of freedom can very well be the cause of lack of quality too, which GNU/Linux shows and will make even more apparent as time goes by and it continues to evolve.
An alternate OS only has a chance of becoming useful if it already has some advantages. Nobody wants to contribute to an OS that stinks, right?
What special advantage does a free windows have over other free OSes? The ability to run lots of old proprietary apps, nothing else. I don't think that's going to attract many more developers than ReactOS does now.
I don't believe windows could ever become something that I would want to use for work.