So by switching to a monthly/yearly subscription, what happens to all those hundreds of millions who in essence paid for Windows when they paid for their OEM computers? There should be a timed limit then, embedded with the license in the motherboard BIOS? Five years following purchase, then you pay a subscription? How are you going to test the market for something like this, it's such a huge scale.
Why re-invent the wheel? Many antivirus have stripped-down free versions of their paid products. If Microsoft wants to get more aggressive with cracks, it can do so at its discretion. Here's
something on KMSpico just from a quick search. Besides, Microsoft largely parted company with Windows years ago to focus on other things like R & D, cloud computing, and Mars exploration.