What you state as MS and Sony's "plan" is basically to move to something that Steam already does. Accessing your library on multiple platforms. Steam already exists as a pretty stable and viable "environment", what Valve lacks is control of their own platform. SteamOS gives them that control.
Valve's initial fears (publicly anyway) were the walls MS started building with Windows 8, and that is what really pushed them to pursue SteamOS. SteamOS by itself could never really become viable solely dual-booting on desktop PC's, so I see launching pre-built SteamBox's as the inevitable next step in expanding that user-base.
I don't care what MS's "plan" is for stagnating game development for another decade. These are problems in the market they created for themselves. I wouldn't be surprised to see MS and Sony consoles running SteamOS (and becoming simply another hardware vendor, rather than a developer of a closed gaming system) in the future.