It would make sense - when your product reaches a certain point in its life, its peaked and starts to become a target for alternatives, and when those alternatives gain enough traction (which often happens "overnight") then your product becomes an also-ran that no-one wants anymore.
Now until recently we only really had Windows, but today we have Android and iOS as serious competitors. What happens on the desktop - generally no-one cares anymore. Microsoft continues to sell fewer and fewer copies and Windows becomes a niche product.
Or they open-source it or just sell it for free and let it take over. they make their money selling Office, Sharepoint and other business tools. I'm sure if Windows server was free, the Linux offerings used in web tooling would disappear overnight. People would start writing websites in ASP.NET instead of PHP, and developers would end up knowing only the Microsoft tools for server side as well as desktop. If they gave Windows mobile away for free too I'm sure they might start to make much more market share.
Remember the only reason Windows is so popular was because it was very cheap compared to the Unix workstations people used to use. Microsoft needs to keep their pricing competitive today too, and that means free.