I dont think they will go to a paid subscription model. That would change them from being the only mainstream OS that costs money to the only mainstream OS that charges subscription fees. ms is losing really badly inthe phone/tablet market. Even their console isn't doing too well this time around ( http://www.techradar.com/us/ne... )
I can believe windows 10 will be the last version of windows and it will just be continually patched. ms sees the writing on the wall and when 10 is out they will be pushing their app store hard to get more pc programs into there so they get a 30% cut. All those sales will add up to more than they will ever get from a single customer in a OS sale. The app store will help their struggling phone and tablet line too.