If I was Valve I would be working on writing something to compete with DirectX and make Steam its own complete platform. Make it so it could run on its own without any OS underneath as well as being able to run on top of Windows, OSX or Linux. Go after the next gen console market with a cheap custom box running nothing but Steam. Just make something that can scale well and work well with a myriad of hardware configurations. Game production is costly, having to make half a dozen versions to support all the systems available just adds to it. This require a lot of support from ATI, Nvidia and the software developers as well, obviously this is easier said than done.