Will iPad 2 users update?
"However, it's hard to say the improvements are truly worth the $500 upgrade price over last year's iPad. If you're still using a 2nd-, 3rd-, or 4th-generation iPad, the Air 2 is definitely worth the upgrade."
I think this is the key takeaway. The iPad Air is a great tablet. The iPad Air 2 is somewhat better but in the same class. I don't think Apple expects a lot of upgrades from people who bought a tablet last year.
iPad sales have slid partly because existing iPad owners keep their devices longer than expected before upgrading. At least as of a few months ago, iPad 2s were the most common type of iPad. If Apple maintains the same rate of new customers it's had in recent quarters and gets people with older iPads to upgrade, it will be in good shape. The iPad Air-to-iPad Air 2 upgrades aren't as crucial.