There's an estimated 20 million Ubuntu users worldwide, probably another 10 million or so of other Linux distros. Not to mention, Linux (and thus Steam OS) is easily installed.
Adoption isn't a problem. Gaming on OSX doesn't work as well as on a PC because you can't put OSX on custom hardware.
> As an aside, Linux gaming would seem to me to suffer the same fate as the Android market times about a billion. Too many possible variations and massive platform fragmentation. It sounds like a development nightmare.
You sound like an iOS developer. It's not like every Windows PC has the same screen resolution and hardware, yet it's the biggest gaming platform...
Have you ever developed anything for the desktop? It's not particularly difficult to get things to run on computers with different hardware...