Re: Don't see this taking off in a big way
Just to second the constant upgrade myth.
As you say, a mid range gaming PC will happily play games for years without needing any upgrades. I typically keep a GFX card for about 2-3 years, then buy the same card again, but by then very cheap of course, and run in SLI, so doubling performance at little relative cost.
And even once you hit the point where new game engines come out, that an existing PC/SteamOS rig can no longer reach max on, then you just drop the GFX down a notch any carry on playing.
If you finally decide you want to get back to Max settings again, or maybe you've just bought a new 4K TV, you just swap the GFX card for a new one. Can't do that with a traditional console.
Also for the first poster, bear in mind that with newer console games, closer to the end of their life cycle (i.e PS3, 360 for the last couple of years), then that notching back of the settings is basically being done for you within the console version of the game engine. As the console is no longer able to play the updated engine at full pace. The engines and the PC(Win/Linux/OSX) move on, the console can't. Hence why so many multiplatform games look so much better on the PC.
This will happen again with the PS4 and XBone, the game engines will move on, and initially they will be able to tweak and improve the performance of the new consoles, getting more and more from what they have. Improving the visuals etc. But eventually, say 2-3 years from now, they will run out of head room to tweak and after that point, there will be no noticeable GFX improvement till a new generation of consoles is created, probably 8-10 years from now.
Meanwhile, PCs, (Win, Linux, SteamOS etc). will continue to be able to keep pace with the updated game engines.
But doing so is the users choice. Either stick with your existing hardware, like the console users, and continue playing the games at the quality that your system is capped at, or go out and upgrade to get the best out the the updated engine. Which with a PC or SteamOS box, is likely to mean just replacing a single component, namely the GFX card.
SteamOS gives you console convenience, but with the same upgrade choice you have with a PC. But this is still a choice. If you choose not to upgrade, games don't stop working, they just don't look as nice as they do on an upgraded box.
Just my 5 cents anyway :-)