If it's running SteamOS (and it will be, even if there's Windows compatibility Valve probably doesn't want to pay for licenses), it might as well be locked to Steam for 90% of people. Epic's launcher doesn't have a Linux version (since they've claimed as have many other developers that the Linux community is 1% of users and 90% of complaints due to distro differences), nor does EA, Ubisoft, etc. They could port it, but by time they had something user friendly and working (which may be difficult since you probably won't get more than Steam Big Picture which doesn't have support for installing non-Steam applications at the moment) Valve would've sold a boatload of these and a bunch of Steam games.
Edit: not to mention anti-cheat, which is a whole other kettle of fish when you have to integrate into a entirely other kernel with different plumbing. Valve's anti-cheat doesn't rely on kernel modules or a ton of client side software at all, so they have a cross-platform advantage there too.
Yeah this is where valve's investment into Linux would potentially pay off. Any platform they make could run Linux and work really well while basically all of steam's competitors either haven't done anything for Linux or have actively avoided/worked against it.
Let's assume the SteamOS would make this device 100% locked to Valve's ecosystem, which would give Valve the ability to sell these devices at cost (by charging fee's on games sold). Let's set the price on the same ballpark of the current consoles prices.
What would make people choose this platform over Windows, PlayStation, Xbox, or Nintendo?
This is where IP comes to play, and why Valve shouldn't have dropped the ball with their game development investments, and they would have exclusive games to get the ball rolling.
Edit: not to mention anti-cheat, which is a whole other kettle of fish when you have to integrate into a entirely other kernel with different plumbing. Valve's anti-cheat doesn't rely on kernel modules or a ton of client side software at all, so they have a cross-platform advantage there too.