The point seemed to be that killing existing Win32 APIs would hurt Microsoft more than it helps the UWP, but for gaming the only thing that needs to happen is a new graphics or peripheral API that's UWP only and is substantial enough for developers to drop Win32 support.
But that's not what Tim said. What Tim said was is that Microsoft, over time, is going to start damaging Steam's current functionality by locking down the Win32 API. Which, given how many Win32 API apps are cash cows for Microsoft or Microsoft's major partners (Adobe, for instance), it's ridiculous to think that Microsoft is going to start tampering with those APIs just to cut off Steam. Even if you believe Microsoft is evil, they're not cartoon villains from Captain Planet, they're not evil for evil's sake, they act according to what they believe is their self-interest. They're not about to start tampering with things their core business relies upon in order to benefit their barely-break-even games business.
