Unfortunately I feel like console gamers are going to switch building their own set-top gaming PCs any time soon. It's hard enough to convince most of guys wanting prebuilts how easy it is, and how much more they get out of building it themselves.
I do think it would be interesting if these building sites start offering boxes with better performance than consoles for around or a bit more money (and then less, over time) and greater performance than the next generation of consoles. But I figure there will still be that apprehension to switch to something most people consider more troublesome (and sometimes it is) than the consoles they're used to. It could be a good bridge between the two demographics though.
For current PC gamers though, it could be a possible outlet for extra parts coming from upgrades. Not bad to have a box like that for certain games in the living room when you just want to lay back and play a platformer or hack-n-slash or (anything produced by Ubisoft) something.