If everyone will jump in console segment then there would be a lot of work for devs as they have to program each game for many console and there would be our dear pc for which programming is quite easier than consoles and gives best eye candy. Also now many people buy 1 ps, 1 xbox and 1 PC but if too many players then there would be many exclusives also means many games missed by gamers.
I get your point. The steam box will therefore have games for both PC and itself. Also, it is possible to make a game buy once play on PC/MAC or Steam box
If it runs on Linux, I don't think there is any need to port a whole game to the new platform, only need to ensure that the game supports a gamepad.