The desktop isn't going extinct. I use desktops everyday, and prefer them for certain needs. Computers are just coming in different form factors just like cars do. Because we have trucks and sport cars doesn't mean that no one buys sedans anymore.
Since I have a nice large monitor I bought I have no interest in replacing it with a touch screen. I have found personally I'm more productive with a mouse and keyboard than when I have attempted some of the same functions on a touch screen. They're great for portable devices like phones and tablets but fail for some, like me, on certain industry production needs. The desktop is still a primary work horse.
I can still just get parts and upgrade a desktop, not so much with tablets or phones. This is what the industry has always wanted. Being able to force new hardware sales instead of part upgrades. I don't know about others but I hate the idea of buying a car that I can't get to the engine on and can't get ANY parts for.