I'm not entirely sure we'll be seeing a wholesale shift to subscription Windows just yet.
I asked MS representatives precisely that question at a recent event in London where Windows 10 was being showcased, and the response was a definite no to a subscription model for consumers for Windows 10, although they didn't say that it would never happen.
There is already a subscription model in place, in a sense, for corporates with a select / volume agreement with software assurance, where the subscription component allows upgrading to the latest version.
Contrast that to the consumer model, where MS provides updates and service packs, but to upgrade to the next version costs ££ (present give-away excepted!).
While MS do seem to want to push all important updates & patches out to consumers without option, apparently, this won't include the next release of Windows. Whether the mandatory update push is the precursor to a subscription model for the next OS, only time will tell.
SD