Tim Sweeney has said this many times, but it's continually not held up. That same month, Microsoft released an update to make it easier to install UWP apps outside the Store. Steam could distribute UWP apps if they wanted to support it.
And Win32 is going nowhere in the next decade, the entire world runs on it, and it's the reason Microsoft is in business.
Tim Sweeney's comments on Windows 10 should almost be entirely dismissed as FUD. We have more pressing issues (telemetry) to worry about.
It's potentially a risk - after all, there isn't and can't really be an equivalent of Steam on Android or iOS. But at the moment it doesn't seem likely.
http://arstechnica.com/information-technology/2016/07/now-ti...