By stack I mean app store stack. If Valve releases Steam for Android you will have to manually get it from them because Google will never allow it.
Yes, but I don't see a difference between this and Steam on PC. An appstore is just an easy way to download apps. Steam can make a special homepage on Steam PC + a badge + a free valve game for every gamer using the app, and the google interdiction will just be nothing.
It is expectations and habits. On Android devices, people expect to get applications from Play / Amazon stores rather than manually downloading and installing an apk. I expect that a supermajority of Android users by a huge margin would not even understand the concept of "download and install an application package" even though they do it every day on Windows. They see Android, they go to the app store for programs.