If you're working with a keyboard and mouse, and looking at a big screen, and you have a rich multi-tasking environment that does what you need it to do, does it matter if it's running on a smartphone that's connected via bluetooth to a keyboard/mouse and via DLNA to a screen? Why the need to have a big box containing your CPU, if a little one will do?

Desktop PC's will always find a niche, but they're already a niche today. Laptops by far outsell desktops, and smartphones+tablets by far outsell laptops. Right now you have to be a geek to buy a desktop, because regular people don't buy those anymore. I don't know anyone "normal" who bought a desktop in the last two years. The idea to buy anything other than a laptop simply doesn't arise. This happened in 5 years. Why couldn't a smartphone-centric transition happen in 5 years as well?