Do you not understand that the GPL has made it so that valve can't do that?
To be concerned about what you're concerned about, valve would have to violate the GPL
Valve puts one piece of commercial software on a completely FOSS operating system, this is nothing like windows, and i'm sorry but you sound delusional. Why would valve make more of the OS proprietary than steam? How could they? If they wanted to, why would they not use BSD?