My only problem with a Steam Box/OS is the same as its ever been, how do they deal with the ever evolving system specs of PC gaming?
Keep it cheap to the point of being almost disposable, or at least tradable, like consoles but at a handheld price point.
Whether you buy a new box, or a new thing that swaps into the box, it's all the same if it's cheap enough. Make consumers think less, and you'll get more sales. It just backfired on Nintendo because people thought Wii U was an expansion. But I own at least 4 versions of Nintendo handhelds, even though there's a certain amount of backward compatibility.