Maybe they can't open-source it because they don't own all the IP? That's very likely the case for Windows as well, for example, Microsoft just didn't licence all the code they used for releasing the source, and now you can't go back to 1000 different IP owners and negotiate anything reasonable.
Didn't they have to manually prepare a binary patch for a security issue in the Word Equation Editor, because they either lost or could not compile the source code anymore?
The original Equation Editor was licensed from a third party (Design Science), and it is possible that Microsoft never had the source code. Maybe the third party vendor lost the source code, but I think it is more likely that getting the third party vendor to fix the bug would have required negotiation with that vendor, and maybe Microsoft and that vendor were having trouble agreeing. (This is speculation on my part, I have no inside info.)
Microsoft probably started wondering internally why they don't just write their own equation editor, but didn't have time, so decided to do a crazy patch to this one and then start on a rewrite.
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.
Here's a kernel engineer from Microsoft answering the question: What do you think about open sourcing windows and getting rid of the licensing code? [0]
The last assertion made in that answer is unfounded and false -
"Even if the entire OS code was made public tomorrow morning, it would take years before someone figures out how to build it, the complexity of the build system itself is mind boggling."
is contradicted by the fact that just recently a version of windows source (old, but still) was leaked, and people did manage to successfully build and boot the leaked windows (xp and server 2003 IIRC) code within days of that source becoming available.
that's what kept Solaris from being open sourced for years.
There's a talk by Bryan Cantrill about that.
They basically could not provide a fully functional OS because some marginal yet used-everywhere parts where licensed and proprietary (Bryan cites the internationalization library as an example).
