Id say a slate replacing desktops is a possibility. Tablets or phones will NEVER replace desktops. The reason is really simple, their screens are too small. If their screens become big enough to replace desktops, they are no longer tablets, they become slates.
Really its not about phones, or tablets, or laptops, or slates, or desktops etc. Its about 2 things. Content consumption and content creation.
Content consumption can be done on any device, size doesnt really matter. Portables will dominate all over content consumption. Really phones are just tablets that can make a phone call. The phone call part is rarely used anymore compared to all the tablet functiosn people use.
Content creation needs a large screen, and probably a static work surface, ie a desk. If you are doing data entry you need a keyboard, because touch is just too slow by comparison.
So, sure the traditional desktop might go away. Probably will for most people. But the need for a 'desktop' ie a not so portable content creation space, will never go away. It will just be called something else, ie a slate with a docking station.