Ultimately, what other choice does Valve have? Every single other company is coming out with its own "App Store" - few are exclusive at this point, but the tide

Ultimately, what other choice does Valve have? Every single other company is coming out with its own "App Store" - few are exclusive at this point, but the tide might turn that way (already I believe Microsoft and Apple have taken steps that have made it more difficult for Steam). That said, OpenGL is an increasingly relevant player due to WebGL, mobile devices, Apple's growing market share, etc. It is also possible to simultaneously target OpenGL and DirectX with things like ANGLE. If your game is written in OpenGL, it is relatively easy to port to different operating systems, assuming you don't rely heavily on platform-specific libraries (and if you already support Mac, it is typically even easier to port to other *nix-based platforms). If the XBox One is OGL capable (via its Windows layer), porting could get even easier. To top it off, I'm pretty sure it will get support from at least of the few big engines: ID Tech, Unreal, Unity, and CryEngine all seem to be potential candidates. 90+ percent of games use one of these engines.