Users are becoming increasingly OS agnostic. They use OSX, Android, iOS, Chrome OS, Windows and (some) true Linux. Enterprise might subscribe, but will consumers? "You mean I have to pay an annual subscription to keep this box working? Sorry, dude but I see that Mint model over there advertising no subscription and machine life updates. Can I do my Internet on that?"
There is one possible exception to my mind: Guaranteed security and stability. If MS says it new Windows will be self contained. That one won't need add ons like subscription AV, anti Malware, or tweakware to keep it running smoothly and safely. And that MS commits to doing all the work to keep its OS in optimal shape, then perhaps, but only perhaps, would an annual fee be acceptable to some. But really they pretty much do that now for free with weekly patches and Security Essentials etc. Moreover, let's remember that Chrome OS does the same hidden maintenance thing for free, too. And better IMHO. Granted Chrome OS is pretty limited, but more and more applications are on tap to work on the platform within Chrome OS and the browser. I also think hardware vendors would see a MS subscription OS as a drug on their market.
The world is moving the other way as the OS is becoming increasingly less prominent. Heck, many people use two or three different OSs and don't even realize it. MS is practically giving 8.1 away to sell its hardware -- as well as that of its partners' -- and to keep market share. Chrome is a giveaway as is Android. I am sure MS would love to get subs for an OS. But it would be a hard sell in today's world of computing appliances. If they couldn't do subs earlier they won't manage now when the rest of the space is in giveaway mode.. And to try would probably hurt their business. What they have to do is make a disruptively cool, kick-ass OS that people have to have to make their new computers do new and wondrous things in the real world (deep learning, AI, robots and smart homes anyone?). They have the resources to do it. Do they still have the vision?