Critique Steve what you will. Point out iPad's lacking features what you will... The point to learn from decades of digital naysayers who proved clueless is:
Predicting how people will use (and how many will buy) is not as easy as it looks. No where as easy as Monday morning quarterbacking. And Apple/Steve certainly have no 100% track record. But they have batted >.500 - in a ballgame (CE & PCs) where batting >.300 is impressive.
Plus... it's naive (IMHO) to consider iPad 1.0 as a static target. With the Release Event behind them you can safely bet they are working on rounding out expanded features for iPad 1.x, 2.0, etc. And (reliable or not) as the following AppleInsider article suggests Apple has probably held back additional features and target audiences:
This reminds me of a point I heard someone make about (I think) a Heinlein novel. Heinlein (or whoever :)) had predicted the widespread use of telephone answering machines. Their point is that this was kind of easy to do - but what Heinlein also did was predict that people would use them to screen phone calls (in the book, the character hears his Dad call in, and picks up the phone and says something like "I'm in for you, Dad.")
The point is that predicting how people will use stuff is the hard part (as you say).
Predicting how people will use (and how many will buy) is not as easy as it looks. No where as easy as Monday morning quarterbacking. And Apple/Steve certainly have no 100% track record. But they have batted >.500 - in a ballgame (CE & PCs) where batting >.300 is impressive.