I definitely agree when it comes to the future of Windows, but it's not like Valve is putting ALL its eggs into Linux. It still has one hand in Linux, one hand in Windows, and a foot in Mac OS. I think the whole point here is that they're no longer tied to a single OS. Now they have an escape route if one goes down, or changes in a way detrimental to Steam.
Also, I still think the main target audience of Steam OS is gamers who aren't already invested in Windows gaming. Overall, it's probably supposed to have a lower barrier of entry compared to getting into PC gaming, possibly even comparable to a console.