I agree with your general point... after all, I'm primarily an Xbox user, but would opt for the Windows 10 version of pretty much everything they release (Fable Legends, Killer Instinct, etc). If it got to the point that I could play everything that would otherwise be exclusive to XB1 on PC, then I'd likely just transition away from using the Xbox in general, which basically means that they'd simply lose my annual Live fees, whilst not actually seeing any additional sales from me.
However... I don't agree that it can only harm the Xbox One either. As the PS4 continues to extend its lead, it becomes easier for third-parties to see that platform as sufficient enough to skip releasing certain software on Xbox, or at least strike favorable deals with Sony for them. If MS is able to make the Win10 store an attractive option as a result of gamers that don't own an Xbox being drawn to it in order to access Xbox games, then because it would likely be considered the same general ecosystem and userbase, it could lead to software being released to it (and subsequently XB1) that would otherwise have not been released. This would however be practically impossible without MS first making the effort to get eyes on their store by releasing compelling software of their own. They've just got to be careful to cut the legs out from under the XB1 without managing to securing a recurring base of visitors to the Windows Store. If everyone simply downloads a copy of Gears, and then returns to Steam then the tradeoff wouldn't be at all worth it... which is why I think the early focus on F2P content that encourage repeated store engagement makes sense.
Xbox hardware (outside of accessories) has never been valuable to MS... but the Live subscriptions (and licensing fees) are, and currently only users of Xbox hardware have been willing to provide that.