I expect they won't do that until Win10 is an established rolling release OS. I also expect they will wait until after Windows 7 is no longer supported (2020) and Win10 has over 50% share. Also, I bet it will be rolled into Office365 subscription. People who have Office365 get recommended and security updates for Win10 for FREE! Such a good deal! You get office, and 1TB online storage for just $10/month!

If they can convert 50% of Win10 users (.625/1.25 billion) to subscription that's $6.25 billion / month or $75 billion / year. Their yearly revenue for 2015 was $93.6 billion. With a billion people (their stated goal for Win10) on a subscription model for $12/mo that is +50% yearly revenue -- enormous growth for such a large company. I do not doubt that is the end goal.

EDIT: "current count" of 1.25 billion windows machines was an estimate by business insider from 2011.