> my counter argument would be that making a Linux version involves more than a checkbox; you now have extra platforms you need to test on and extra complexity in supporting your game in return for very little benefit.

I don't think that's a counterargument, I think you're agreeing with what he said ;)

> If steambox provides a known Linux environment with users wanting games it becomes a much better proposition to make a Linux version

That is a counterargument. I tend to agree. I hope you're right!