Linux is all over the place. I know plenty of people who use it daily. I lived on Linux for years. Hell, I even ported the Opera Web Browser to the platform.
What it boils down to is simple, OS wars are dead. There's more than just Microsoft now. I personally prefer Windows 8 because it's faster than anything I've ever used before and it has less obvious bugs than the other platforms. Other people like Mac, others Linux, others Chrome (which is more of a Java platform than a Linux platform).
I think it's about time to consider that 99% of game development has moved into a new era of platform independent game engines. Using Unreal Engine, Unigine Game Engine, Unity3D and others you write the game once and tweak the controls for a dozen different platforms from phones to XBox/PS to Linux. Companies who code their own game engines and want to reinvent the wheel can do so if they want, but honestly, it's not so interesting. These days, if a game system developer really wants their platform to take off, they can make agreements with the platform system company and pay for the port or do it themselves.
Take a look at Microsoft. No one wanted to port to the Metro platform and Microsoft basically made it happen by working with the game engine companies. Now all the game vendors need to do is simply generate a new executable and tweak the controls.
If Valve wants support for SteamOS, the answer is simple, port the game engines. But I have no interest in games locked into a platform. I stopped buying consoles because I don't need a special machine for games anymore. Windows, Mac, Linux, iPhone, etc... are all powerful enough to play the best of them. Game consoles were only interesting when porting to a platform meant an endless amount of problems with hardware incompatibility. We don't do it anymore. These days, the game engines do the work for us. Content developers can produce awesome games without worrying about AMD vs. Pentium or nVidia vs. Intel vs. AMD. Hell, they don't even have to think much about Mac vs. Windows vs. Linux. They can develop games and simply deploy them.
SteamOS seems interesting, but I want one device for everything. I use a Surface Pro at the moment. Surface Pro 2 later this month. It's a laptop, a tablet, a video player, an ebook reader and a game system. Would I like better graphics? Yep... but Pro 2 has better graphics. And the graphics on the Surface Pro 2 are good enough that it's now more about game content than graphics quality. I carry an XBox controller in my backpack so I can play Sonic Racing or Lara Croft on airplanes.
It'll be pretty cool though if Valve makes it so I can buy a game and play it on SteamBox or my Surface without buying a second copy.