I think that Microsoft can open-source most of Windows sources. Nobody would care too much about few binary blobs and I don't believe that they don't own license for a significant portion of OS.
This is exactly what happened with Solaris, and it turned out to be a rather massive problem because it meant that the community couldn't actually functionally produce a derivative distribution because the original released source code didn't actually represent the entire distribution. And a project that the community can't build will always be critically undermined by that flaw.
I think that momentum behind Open Source Windows would be immense so community would overcome any problems. I mean, people are making Windows distributions right now, with all sources closed, and they're making amazing work if you ask me, with all those reverse-engineered knobs and whistles. Solaris is niche OS after all unlike Windows.
I dunno if I'd call it "solved" or not... Illumos reduced the binary blobs, but to this day you have to download a bundle of them when you build it. The whole issue also added significant friction early on, which I personally think stunted the project's growth, but I'm not sure that really knowable.
