The desktop as a form factor will never go away. I know for me personally the day I need to turn in my machine (including keyboard/mouse/large monitors) for a smart phone or tablet is the day you can go ahead and just shoot me to put me out of my misery.
Monitors and mice might be replaced with a device which tracks your head and eye movements and paints an image on your retinas with a laser. Keyboards could be replaced by pre-speech recognition, measuring the impulses sent to your jaw and mouth muscles, probably in the form of a headset.
Your "desktop" computing needs are filled by a headset which occupies your entire field of view with its environment, and your "mobile" computing needs are filled by the same headset, but it only occupies the periphery of your field of view. When you sit at your desk and face forward, the environment goes full-screen, and when you look away or go away from your desk, the environment retreats to your peripheral vision.
At the end of one day of using that my neck and back were so sore I could barely stand to sit at my desk!