Microsoft wants teh annual revenue stream, but the marketplace has become so competitive with versions of Linux and people willing to use older versions of Windows and alternate paid Anti-Virus they have themselves painted into a corner, and until they can make some new amazing features they can charge a yearly fee for they will never get that Windows as a service model off the ground.
They really did themselves in (for me and a lot of others) when they killed off Windows Media Center, it had the ability to run on a second screen while still using windows for gaming, I loved that feature, my wife could watch netflix or movies while I was gaming on another screen with no noticeable effects.