But that's not what they are doing. They are coming up with an OS that will work on most PC hardware. The OS can be downloaded by people savvy enough to install it on there own. The Steam box is for the "console" gamer who really like GameX that is only available on Steam. So they buy the box from Valve.
The question really becomes whether or not the game developers will actually follow Valve into Linux. If they do I can see some PC vendors offering a computer with the option of the SteamOS much like Dell has done from time to time with Ubuntu. Remember they can still sell the same hardware as a Windows machine so it's not like they are stuck with hardware that can only run SteamOS. What else could you have used 3DO hardware for at that time? I see this as much less risk. So it hasn't quite been done like this. But, I do see your point.