I don't think win10 will necessarily be subscription based but I could see it where they force you to tie it to a central/cloud account of sorts, like phone OSes do. Will also turn into a very closed system that is full of ads, tracking, and other crap. I put up with it on a phone because I don't really have a choice and don't really use it for a lot of things, but there is no way I'd want to put up with that for a computer OS, which is why I now use Linux.
I also think it will eventually become adware, all the tracking they do is basically to have enough data for when they start their ad platform. You will be able to get windows 10 for free forever but there will be ads, but you'll be able to pay to get the ad free version. Really it makes total business sense. It's one thing for Google to be able to advertise to you on virtually any website that has adsense, but MS will 1 up that by being able to advertise to anyone who has a windows computer, period. That's an advertiser's dream. Basically when you open a program, you'd occasionally get a 30sec commercial come up for a related product. After the commercial you'd then have the option to continue to launch the program you were going to use, or to buy this new program through the app store. They could also tie in with Amazon and/or other major retailers and such so when there's a commercial or ad for a physical product you could set it up so you can just click "buy now" and it automatically orders it for you. The possibilities are endless.