Windows [kernel] is a very advanced operating system with an incredible wide support for third party drivers. I think the prediction would be more oriented to running them side by side.
Take this with a grain of salt, of course.
http://www.kuro5hin.org/story/2004/2/15/71552/7795
I'd say, the user experience has improved since then overall (meaning the code quality, too, in one of the most practical senses).
You get dodgy comments in the Linux kernel to.
Apple's non-unix line endings (CR) changed to LF, their non-unix path delimiter (:) changed to /, and they were suddenly a major force among non-Apple-targeting developers who had always derided the Mac as a "toy" prior to OS X.
If MS has similar ambitions, they could do again what both they and Apple have done before: toss away millions of lines of kernel code.
That was because with OS X, they switched from a Mac OS subsystem to a Unix subsystem
Current-generation Windows doesn't really have any such flaws-- the underlying kernel seems to be pretty sound, the hardware driver ecosystem around it is the most complete and robust of any mainstream OS, and they seem to be showing now that they can adapt to developer demands without doing something drastic. There's no real compelling reason that I can see for Microsoft to make a transition to Linux (or BSD, if the GPL is too toxic).