My guess, based on Proprietary Codebases I Have Seen and the apparent general philosophy of Windows development, is that people would react to the now-open-sourced Windows code with either hilarity or horror (or both at once). There would be critical articles in the tech press. Then a small select group would mine it for low-level interoperability, but none of the code itself would be retained. Emulation layers such as WINE would end up being able to reproduce Windows’ quirks more thoroughly, but finding the important bits in a mess of Someone Else’s Code would slow down development as much as having an exemplar would speed it up. This all assumes that the code was released with an acceptable license.
On the Linux side, mostly a wash beyond some small interoperability gains, in other words. What would happen on the Windows side, I wouldn’t venture a guess on.