While home PC sales have fallen, PC gaming is still fairly strong, with companies like nVidia, ATI, etc still coming out with newer and faster technology.
QuakeCon this year had more participants and others in attendance in its over ten year long history. Big games like StarCraft 2 and such are still selling millions of copies. Chris Robert's Wing Commander startup "Star Citizen" has blown everyone away and exceeded their goals and stretch goals every time they set them. They started at 2 million, and now after raising the bar umpteen times they are closing in on 20 million in donated funds from the gaming community.
So, I agree, crappy home PC sales may disappear, but custom and gaming PC sales aren't going anywhere. Having had a look at some of the new console stuff coming in the fall, it still isn't as good as current PC games, much less future ones. Check out the number of units and complex stuff going on in Rome 2 when it is out next month, and you'll see what I mean.
If consoles ever get it together on the controllers and offer a keyboard/mouse option that isn't just a gimmick, but a really well designed setup supported by the games and the consoles, then you may see PC gaming be cut to the bone, but even then, i doubt it.