I agree. People upgrade tablets much less often than a phone or a laptop. The product category will eventually become like the iPod IMO.
The iPhone replaced the iPod, there will need to be a product to replace the iPad. The only only product that I think will replace the iPad is a MacPad and will that really be different than an iPad?
Certain devices were born to fail, the iPod was one of them it was a stopgap product until the iphone came. I just don't really see anything replacing a watch, phone, tablet and a woman's purse. Maybe google glass can, but I don't think that will fly, then again I have never tired it.