This is true. A quick example would ZIP Folders in Windows - it uses third-party code Microsoft licensed (who knows if the company is still around today since it dates back to the early 2000s).
There might be better examples as that could be rewritten to use an open-source library such as InfoZip. Or simply stub-out the licensed functions and let the community fill it in. (And with something better, such as 7-zip so we get ZIP, RAR, 7z, tar, etc. folders.)
But I don't doubt there is other code that is not so easily replaced.
I’m not sure such libraries could be rewritten (in a white-box, non-IP-infringing manner) while maintaining the bug-for-bug ABI compatibility that Microsoft (mostly) guarantees their customers.
Yes - and if Microsoft announces they're open sourcing, or that information leaks, the owners of the IP will be able to name their price. Each will be trying to hold out to be that last essential component to get the highest price for the new rights.
There is, of course, always the Java approach: open source everything you can, on the assumption that that is 99% of what's needed, and allow others to fill in the gaps.
I don't know that Windows would run without the licensed code, or at least Explorer. If they didn't bother to write their own ZIP library (or use an open-source one) then they probably licensed a lot of other stuff that one would expect a first-class desktop is to have, making it unusable without a bunch of developers fixing it (when their company would probably buy them a license anyway).
If it was released without the third-party code, then it'd be a research project, which'd be pretty neat but not something most probably expect from "Windows is open source".
Java, when open sourced, couldn't run with purely open code (it relied on some binary blobs Sun provided); it's plausible MS could do similar. There would likely be a fair bit of interest in replacing the binary blobs in Windows.
Unless that piece has some sort of IP beyond a license of the code being used they run a real risk of Microsoft deciding it's cheaper and easier just to rewrite their own version of that library.
But I don't doubt there is other code that is not so easily replaced.