Tablets will never replace a desktop machine, the graphics, and pure power both in Ghz and usage just aren't comparable.
Tablets are great for general usage but can you really see COD, Halo or Diablo being half as good on a tablet as they on the PC? Not in this lifetime.... Angry Birds is about as good as it gets
But then how many people play COD, Halo* and Diablo on PC? Most will play them on games consoles.
The point is Desktops will never disappear entirely, however their marketshare will decrease (and probably already are decreasing).
*You play Halo on the PC?! Shame on you, same with those that play GTA on computers, both are console games and should be played on consoles. The reverse for COD, it is a PC game, it shouldn't be played on consoles!

the way I see it long term:
Desktops - Every day home use
Laptops - Will get phased out for hybrids for every day use and taking to office/mates
Tablets - Occasional browsing and using on a train or something for youtube/iplayer
Smartphones etc : Portable browsing
all of them have a purpose, but nothing will replace anything, apart from the gradual laptop-hybrid change.
For you maybe, not for most people. Laptops have already taken over as the main computer in most houses.