Well, DOS is very old and incomparable to modern systems in regard of comfort and program availability. Therefore Microsoft has little value from it. On the other hand, Windows has lots of value. Because even when most private customers get recent versions as free upgrades, the corporate site looks very different. Just think about different server versions of Windows, which most possibly share huge parts with the Windows 10 kernel. They cost real money + user access licenses. Therefore - although I hope you are right - I don't believe that Windows will get open sourced anytime soon. I like the idea very much, but I'm trying to be realistic about this.

Is there any info on how much revenue is extracted per software item? Windows is most widely used one, but I am sure it is not one that extracts money the most.

MSDOS v2.0 is about 52,000 lines of assembler code.

Windows is in the order of 40,000,000 lines of mostly C++ code.

It is very unlikely that MS would release intellectual property that is approx 3000 times the size and possibly a several million times the value of MSDOS.

Even assuming that they did, it will take developers a long time to come up to speed with understanding the code base sufficiently to make useful changes. In the meantime MS won't be standing still.

There are ReactOS developers who do understand Windows. They would be happy with increased user base and community support.