One look at Apple’s App Store revenue graph gives a pretty good argument why Microsoft would want to adopt this model.
I think both Gabe Newell’s and Sweeney’s thoughts are a realistic estimation of the situation - long term, of course.
For example, is their platform open enough to build something like Steam on top of UWP? Of course not, apps that install other apps are a security concern (operating systems that install apps and prevent us from removing them are ok, however :D). If they simply push UWP hard and reposition Win32 as legacy and/or server platform they will already hurt Valve.
The sad part is that if this future comes to pass, app stores will limit to curated, filtered and sanitized content. Someone wants to publish a thought-provoking book/movie/game that satirizes religion? not on our app store, it might offend people. Question moral standards? Nope. Criticize a government? Nope. Use erotica to drive a story or push boundaries? Nope. An inconsequential time sink that tries to rob people via IAP? Yes, you’re welcome.