They could always do a "partial open source", where DLLs with license problems are kept closed-source. There's precedent for that; I recall a successful open sourcing of an old game engine where the third-party audio library it used couldn't be released.
In fact, I could argue they are already slowly going a "partial open source" route, with their gradual open sourcing of parts of the dotnet runtime.
Exactly, pieces that would be too difficult for them to Open Source I think communities would gather to build those out to form a complete Open Source Windows.
In fact, I could argue they are already slowly going a "partial open source" route, with their gradual open sourcing of parts of the dotnet runtime.