Alienating their main user-base for the sake of promoting their own console seems like a high-risk play when most people already are aiming at spending their "console-allowance" on the PS4 or One. The Steam Box is supposed to be modular (upgrade-able), while being strictly plug&play, this alone gives it a huge edge over existing consoles. Now we know SteamOS will be Open Source, and we know both Nvidia and AMD are commited to improving Linux drivers. What that means is AAA-titles don't even need to be ported, the Wine layer is able to provide 1:1 performance as proven with for instance World of Warcraft. The Wine devs don't even have the luxury of source access like game developers to.

All in all, if Valve does this right they're sitting on a potential goldmine.