I'm pretty sure "can't" in this context is legally binding. Windows licenses up to this point have been sold without expiration dates. If Microsoft suddenly started charging a subscription to keep using the same copy of Windows, evey law firm on the planet would jump on that in an instant.

What GP proposed is the much more likely avenue they would take: New version of Windows with a new licensing model. It would probably kill their consumer business overnight, but at least it wouldn't get their lawyers laughed out of a courtroom.