The problem is that iPads keep on going, and going, and going. They're incredibly robust. They start with a long battery life leaving plenty of room for battery decay while still having useful run times. Current apps generally run fine on older devices. Even when they don't an old iPad can still perform many useful functions such as email, video playback, maps, reading ebooks or pdfs, web browsing and slightly older or casual games.

It's beginning to look like their average useful life is far longer than that of a Phone and also longer than a desktop or laptop. As a result the replacement rate may be fairly low. I'm in the market for another iPhone this year due to contract exipiry, but my household has reached iPad saturation. The kid's Minis are still going strong and we bought an Air last year after giving our '2' to a relative and it's still going strong too. It's hard to see when or why we'd need to get a new ipad as long as any of our current ones still work. That's probably another 3 years at least, possibly 5.