> "All of this insulates them from having to worry about the long-term sustainability of making money from game sales on Windows..."

The weak link in your theory is that Microsoft is in control of the future of the DirectX API, not Valve, and it is Microsoft who is working with nVidia and AMD and game studios to evolve DirectX to take advantage of the latest GPU features. SteamOS can at best follow closely behind but can never take the lead without Valve developing their own games API that games developers an GPU makers are willing to target.