I don't really see the market for this thing.
I feel like the average casual console gamer has no idea who Gabe Newell is or even what Valve is. Those who already know who he is and follow what goes on with Steam are either hardcore console gamers (the minority) or PC gamers (another minority). I'm trying to imagine what angle he could use to advertise his Steambox to the general ignorant public that would convince them to move to an unproven new console versus buying the established, familiar Xbox or Playstation.
What exclusives will the Steambox have that would bring over people from the other consoles? There aren't that may popular PC exclusives that aren't MOBAs or RTS games. Most people have decided if they want to play those kinds of games already and have purchased their console accordingly.
If you have the ability to install Windows on it and do PC things with it, why not buy a regular PC (prebuilt or self-made)? What makes this the better option? The only positive it has going for it is the absurdly good Steam sale prices for games, but you can access those already with a regular PC. Also, if the Steambox is upgradable, what is the difference between it and a regular PC to begin with?
I'm not seeing the appeal of this Steambox versus a regular gaming PC.