But that would be Steam OS exlcusive, not actually on any other Linux distro. So while it might trickle down to better game dev tool support in linux, it wouldn't on face effect adoption rates.

If you say that SteamOS means a game is Linux exclusive, you can say Metal Gear Solid for was CellOS/BSD exclusive[0].

[0]http://en.wikipedia.org/wiki/PlayStation_3_system_software