Just Like the 'Paperless Economy'
Does anybody remember the predictions that the world would transition to a paperless economy where pen and paper would be obsolete and we'd magically save the world from the horrors of deforestation? That hasn't really panned out yet, has it?
Does the tablet offer advantages over the desktop PC? Yes, namely, it is more portable. That's about it. Is it faster? No. Can you do more with it? No, quite the opposite. Is it customizable? No, not at all. Can it be tailored to the needs of the user? Nope! One-size-fits-all!
Will the tablet replace the desktop PC? Only if we want it to. It's this sort of needless prognostication that ends up making people look like fools down the road.
The large size and modularity of the desktop form factor make it a better choice for people who need the power or who have special requirements. I think the big thing the tablet has going for it right now is the complexity of the software required to take full advantage of a desktop PC. There are very few applications other than games that really tax desktop PC's. Of course, that wasn't the case only 5 years ago (remember the transition to Windows Vista?) and likely will not remain the case as software continues to progress.
I expect that the next major developments that will push power requirements enough to require a Desktop will be 3D user interfaces and alternative input devices.