The answer is that nobody will support it as-is... they would just use the source to make or improve other things. Because this is what would be in the interests of the users. Even if it meant Windows got buried.
But this is strictly imaginary because I doubt that 20 million lines of code are all under the unilateral control of Microsoft.
I think open source is great and I even admire Stallman because of not despite his single mindedness. I also don't think Windows being closed source is problematic. Choice is good and Windows is able to solve a particular set of problems efficiently by virtue of Microsoft's business model.
That model fit my needs and interests for many years. Less so now because my needs and interests have changed to where the tradeoffs Linux and Open Source impose are outweighed by their advantages. But other people have other needs and there are already enough first world problem zealots in the world.
>Everything you can do with closed source software you can also do with open, and there are things you can't do with closed source.
This is just not true. Show me a completely open source smart phone... thing is you can't, the baseband and radio will always for sure be closed source. There are markets where having knowledge that your competition doesn't is vital to success, that is why some things will always be closed source.
A closed source project can license closed source technology and save development costs while avoiding putting 1.0 code for an edge case into their core product. The exchange of money helps keep the interest in support, maintenance, and extension by those who first developed the technology.
Open source projects can also release v1.0 with bugs, it's not like there is some Open Source Police stopping you from doing that. You can also pay for support of open source software, and that benefits everyone and not just the company that happened to have money to invest in it.
There's still money licensing Fortran libraries that have been around for decades. They are fast and good and cheaper than achieving the same levels of fast and good from scratch. There's nothing intrinsically wrong with using some gem written over a weekend of craft beer. It just may not make sense to bet a business on it even when you can read the source code
StackOverflow is an example of why the choice is beneficial. The source code being proprietary is irrelevant because the content is Creative Commons.
But this is strictly imaginary because I doubt that 20 million lines of code are all under the unilateral control of Microsoft.