Well, it is all still speculation so far, but Paulo and I have been discussing it. 10 will be the last proper OS (which is to say it is a hybrid of 7/8 anyway), then from then on they'll be just updating the OS rather than a full release. When I first read the article on BBC News, Microsoft said something like "Previously we got our engineers in a room and they spent 3 years working on a system, but times have moved on and now we spend ages always adding updates anyway, so from now on any new Windows will come as updates".
We also can't see then forcing people to subscribe to Windows (which isn't a guarantee that they won't), but if you think of all the manufacturers and all the retail sales, then it isn't really sensible to get people to subscribe to get an OS, as how would you do that when you buy a PC? Plus if they try that, someone will make an equivalent OS and cripple their market. BUT we do think that they'll charge more for various bits and pieces - Skype, Office, Dropbox, possibly Spartan etc - and make the money that way. Remember, Microsoft mostly make their money from Office, so charging for Windows is pointless.