Since every PC running Windows 7 onward is going to be eligible for a free upgrade to Windows 10, I honestly think that it is quite possible to see a billion devices. I’ve got 11 of them to upgrade myself when the time comes.
Considering the vast improvement in quality of use that came with Windows 8.1 and appears to be expanded in Windows 10 - it seems an entirely achievable result. A plus being constant updates from then on out without having to buy a new version with each iteration.
Sure, Windows Phone owners will see an end of life for their devices. The same that Apple and virtually every other phone OS builder has; hardware moves on. As for desktops, I’m sure there is an end of life there as well - as memory and graphics expand, so shrink the number of compatible devices. But since Windows 10 seems to love to operate on a wide variety of platforms, it would not surprise me if it extends beyond the ‘operating life’ of how long XP lasted.
None of those 1 billion devices will be in my home.