I think if you were able to look at the current source code for Windows (including 8.1), you would find that there is a lot of code that is unchanged or only lightly modified since Windows 2000.
It's not like they started with a blank slate when they began working on XP and Server 2K3. Or even Win8.
So they would be opening up source that is still part of the current code base.
I also dread to think what would happen if the did. Win 2K isn't so ancient that the open source community couldn't take it and make a usable in 2014 OS out of it. I'm sure it would be done. More than once. And you would end up with the tangled mess you have in in Linux world, where you have a whole bunch of different flavors of an OS that are almost but not quite the same. Yuck.
I suppose they could release DOS or Win 3.x. Although I bet there are fragments of that code that still remain here and there. And who would want it?