Quantum Break is no longer a Xbox One exclusive. It is releasing on Windows 10 at the same time. The claim that this will kill Xbox or that there is no longer a point in buying a Xbox One is now rampant in the comment sections to IGN and Polygon articles about this news, on Twitter, and all around the internet.
It also exposes people for having no idea what's actually going on.
If there's no point in buying a Xbox One because a Windows 10 PC can (or will eventually be able to) play any Xbox One game, then by that logic, there's also no point in buying a Windows 10 PC because a Surface can do everything that a Windows 10 PC can.
But that's exactly the point. For anyone paying attention, Windows 10 is suppose to be a service that runs everything, on any computing device, of which Xbox One is a subset of the devices that runs Windows 10. And there is no particular reason Microsoft should be concern that people choose, as a result of the non-exclusivity of Quantum Break, to not buy a Xbox One, which is a Windows 10 device, because they will then require a Windows 10 PC, which is another Windows 10 device, to play the game. You are just substituting one Windows 10 device for another Windows 10 device.