It might be nice for a little while, but the PC component is going to age much more poorly than the display will.

I think the better move would be for Valve to make a really nice gamer-oriented dumb TV that's essentially a 50"+ monitor. Kind of like those BFGDs (Big Format Gaming Displays) sans the exorbitant prices. The size of a Steam Box is in comparison quite diminutive, so finding a place to put it shouldn't be too much of an issue and the ability to swap it out for a newer model with the same screen 5+ years down the road would be nice.