A lot of things unsaid behind the scenes...
Yes, MS could suddenly start invoicing for "upgrades", patches, etc.
Or as has been pointed out, they want your data for advertisers.
By pushing this to the PC and then the tablet market there's a foot in the door for the advertising model. All that's left is the phones... get them and they have you as a captive audience for whoever flings cash for ads at them.
It's also possible that MS is looking at a combination package.. "don't want ads while working in Word or anytime we feel like pushing some at you... here's an invoice each month but we'll still suck your data for when you're browse the web."
Yes, they need income, and while their execution on things like phones and tablets has been less than steller, I suspect they've learned a few things. Plus, with Ballmer gone, there's a bit room for a grand plan instead of a reactionary one.
Like a lot of others here, I'm holding fast on Win7. Once I see where this is headed, I'll either go with Win10 (as one should with any "new" OS from MS) or go with Linux or (god forbid) a Mac.