I think PCs will always be around because of the ability to modify the hardware and have better power sources for whatever it is you want to do: gaming, graphics, etc. Mobiles, including most laptops, as nice as they are don't offer the kind of resources needed for serious computer usage.
I think we need to understand that in this day and age, everything that is present and familiar will be replaced with something that will not only fill that purpose but do it in such a way that is more convenient and better-performing. People are still reeling that CDs are a dying technology (Heck some still haven't given up on the cassette or 8-track). Smartphones and tablets have penetrated technology into places one would, ten years ago, never have expected to perform computing.
So are desktops dinosaurs? I think we need to rethink the desktop as a tower, keyboard, mouse, monitor, and maybe some speakers. I think a workstation is the eventual evolution of desktops until computers become completely absorbed into television. If one's computing and entertainment is better met in a comfortable recliner with an AirMac or a decent tablet, that is the evolution we may move to next. Mobile computing with laptops already gobbled up much of the desktop's market share, and this will most likely continue with improvements to the power, screen size, weight and battery life of mobile devices. Just like those Neil Diamond 8-tracks, though, desktops specifically will not vanish overnight, as people still find value in modifying and upgrading their own to save money, and can keep up easier with ever-changing benchmarks in computing.