My iPad 2 isn't going anywhere till at least 2016. Its a great tablet, still does pretty much everything that an iPad Air does, just a bit slower.
Yeah, I upgraded from the iPad 2 -> Air last year; I really didn't need to, but I did want to; partly for the Retina, but mostly for the much faster connectivity. With a decent, modern router with MIMO, the Air is a lot faster.
Who knows how long it'll be before Air owners would need to upgrade. By comparison, I have an iPhone 4 that I can't wait to dropkick into oblivion as soon as I have some spare cash to cover an iPhone 6.
I think Apple will really need to push games in order to drive upgrades. It can't get much more Retina. Speed even in the Air is great, very few apps stretch it. The only other thing is they keep pushing progressively more hardware-heavy iOS updates that make buying new hardware desirable. Which is one of the reasons I don't (major) upgrade my iOS beyond the first year or so.