I would have to agree with the above comments. There are waaaaaay too many people who have desktops/laptops and all they use it for is to check email. So for those types of people I would suggest a cheap netbook, but if the purpose of the iPad is just that - to surf the web, read ebooks, etc - then that would be perfect (other than the price tag, seeing as how I can get a laptop for less than that).
I honestly do feel that desktops are really only for people who, well, need that kind of power and space - programmers, designers, gamers. But because of that, no, they never will die out (of course I guess that brings back the truck analogy). The people that need them will still use them (and the people who THINK they need them will also still use them).
And since we're talking about apple, I've always held the notion that apples are well suited for people who aren't too tech-savvy. The problem is that since that demographic is the way they are, they don't know enough to realize they should be getting that OS. But, obviously, that's just my personal opinion.