Then why did they change the NT version from 6.3 in Windows 8.1 to 10 in Windows 10? Initially, I think that was the case but they later (I think it coincided with Ballmer leaving) decided to do exactly that--hence the massive jump in version number.
I think we will see a Windows 10.1 and probably a Windows 10.2 but I can't see Microsoft giving away free upgrades forever. Mac OS X works for Apple because the upgrades are paid. If Microsoft moves to that model, it will stymie technical progress which is exactly what Microsoft doesn't want. I don't know when and I don't know why but I, too, agree that there will be something after Windows 10.
I can't deny that it is a possibility. At the same time, let me propose an alternative.
Engineer A: We've got to choose something other than 9 as our version number because we've got spaghetti code reaching so far back that it was designed for Windows 95.
Engineer B: Why not just rewrite the code, and implement all the things we've learned in the last two decades?
Engineer A looks at Engineer B, and both laugh.
Engineer B: Alright, We'll go with 10.
Engineer A: But what about all the other stuff? I mean, having fundamental portions of our code referring to completely different revisions is just confusing.
Engineer B: Let's just standardize everything to 10, so there's no questions.
Engineer A: Sounds good. What about all those other revisions numbers?
Engineer B: We'll have a bigger gap in numbering, so legacy code is easier to see and obsolete. This is supposed to be a perpetual OS, right.
Engineer A: Sigh... I guess.
It's admittedly boiling a massively complex numbering scheme down to a simple conversation, but I'm going Occam's Razor here. The less someone has to remember, the better. The more overlap, the less memory required. I see this as a PR bonus (putting distance between Windows 8/8.1), but not a main motivator. Ballmer isn't going to be fondly remembered, but having enough spite to drive a complex project to switch numbering schemes out of spite seems...excessive.