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.
anticensor|7 years ago