And yet you don't buy an iPhone as a replacement for your computer. People still need a "real" computer, and for that reason we're probably not going to see computers go the way of the dodo in favor of iPads.
The computing device that takes care of 80% of your day to day computing needs? (ie: Watching videos of kittens on YouTube)
And heck, even if that's the case and the "real computer" turns into an incredible niche device, do we even need the power of a gigantic desktop machine with 8 gigs of RAM and the super duper video card? I don't know about you guys, but all I need is a terminal and I can do 99.9% of the hacking I want to do (sometimes I use gitk).