Then they would simply buy a computer.
No one who plays in the console market exclusively is going to want this. It's a computer you plug in to your TV and comes with a fancy gamepad. You can already plug a computer in to their TV. This thing costs as much a computer and acts like a computer. Will a Linux based OS be more stable than Windows? Sure. However, it's still an OS on a computer. It's still the same as what you put in to a desktop, just in a smaller case.
The reason people justify an expensive computer is that it does things other than just video games. People use them or work and other things. Gaming is simply an added portion, thus, it's justified. The Steambox, OTOH, is being marketed as a console replacement, primarily for video games. But it's too expensive. This is a dumb business move by Steam and there really isn't much of a market for it. Anyone who is that interested in PC gaming is going to want to build their own.