You stated that Ubuntu is an independent distro that doesn't need the ecosystem and therefore should screw it and this can't be farther from the truth. Without the ecosystem Ubuntu is nothing but a wet dream of Shuttleworth, but you are too much fanboy to see that, I guess.
Valve has its plane about jump in the player's living room without pass through Microsoft or Apple OS. Valve don't care about ubuntu and the proof is that:
1) They already had a working demo in house for several months but they started to accelerate when? When MS has released win8. Ubuntu did not change in that time, so basically ubuntu doesn't play a role for the valve's final decision to put all the effort in the linux project.
2) They working on the steam box that does not ship nothing written by canonical, not unity, not usc and not Mir for sure.
3) They will continue to use X for the following two years as a minimum
4) they targeted ubuntu for the internal test for the same reason ubuntu choose debian as a base: praticity.
5) Steam is now avaiable for all distro and works pretty well without ubuntu, it means ubuntu is all except necessary for them.
6) Ubuntu can force Valve to do A instead of B? Yeah, in your dream only.