It's the move to Windows as a Service (WaaS). New OS features are going to come in the form of Windows Updates. While I'm not saying Windows 10 will be the last OS Microsoft produces (as some have speculated), I believe that Microsoft will be moving towards a more web/cloud based OS (think Chrome Book). They have started this with Microsoft Accounts (vs. local accounts) and this is a continuation of it. Office 365 monthly subscriptions are a stepping stone to paying for your OS per month (this is a long ways off in my opinion, but it is feasible).
"If you aren't paying for a product, you probably are the product being sold." This is how I see the Windows 10 upgrade.