I don't think you can strip Windows' importance either... first, its the most used computer OS on the planet, no matter what Windows OS it is. Secondly, a big part of people are too used to it that they won't travel to Linux or Mac, and of course, speak about it having the biggest program base out of the three main OSes. Thirdly, people who will jump from Windows to any Linux distro be it SteamOS itself will find a learning curve, this learning curve is what makes Windows 8 bad. And finally, I don't think MS wanted to restrict program making, otherwise the traditional desktop would've not been there with usual .exe installing, right? If they wanted to do that, they have to make it exactly like Apple's iOS; apps from outside the appstore are thrown out of the window till you jailbreak. No, you get the desktop and can also install programs, so the developer fuss against Windows 8 being more closed is... I don't really understand it. (Unless its of course, the Windows 8 exclusive games and apps)
Those who will get SteamOS because its made by "Valve" might find themselves getting back very soon to Windows because of the learning curve, I hope they do so because Valve's efforts are directed to the wrong way, and so, these efforts must die miserably to serve a lesson for listening to the entire base rather than a part of it. Because a minority only seems to be interested in Big Picture.
And even if they do Half-Life 3 as an exclusive on it, it will be developed in Linux, so basically any Linux distro would run it. It will serve nothing.