The source code of Windows XP is widely available. Same with a ~2 year old version of Bing, Bing Maps, Cortana etc. Yet that doesn't seem to have had major negative effects on those products. If anything having the Windows source code available seems to be a net boon for Windows development. Sometimes looking at the source is just better if the documentation is unclear.

MS probably hates that the source for XP/2K3 leaked because it means more people will put in effort to fix and extend/backport, even if it's not truly legal, when MS would rather coerce them into the latest most invasive and user-hostile version. Also because projects like NTVDMx64 show how some of their decisions have been political instead of technical as they like to claim.

Far less people care about Bing or Cortana.

If you compiled it and the resulting binary was substantially similar to the original you’d likely get sued.

The limitation is the amount of money & political power the owner of the software you're cloning has.