That's the tricky part, if these devices aren't locked to a proprietary OS/marketplace, users can shop around and you're basically helping your competitors while taking all the losses.
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.
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.