Tablets were never going to sell like they were forever. We're approaching the point where most people who want them, already have them.
As I read the reports, most of the tablet sales for non iPad devices were filling the channel. The reports always described sell-in, no-one ever reported sell-thru rates, except Apple, for which they were 150% or so. When all you are doing is stuffing the channel and no-one is buying, the sell-in rate will drop like a stone once all of the stores have their stock. Later at the fire sale the sell out will be dramatic but your profit is a large negative number. That story describes the so-called tablets. Perhaps in the future there will be competition for the iPad, but it isn't here yet. And yes the iPad is a very useful tool for lots of people, that's why there are stories every week about some company or other buying thousands for some project. It is not a replacement for a MACPro, but then again, it was never intended to be one.
And as for the argument that you could do it all with a netbook/notebook/desktop/??? yes and you could probably also use a pencil and paper for most of it and your imagination for the rest, but would you? And would you want to?
My netbook was a fine computer but could not replace the macbookpro on trips. The iPad has.