One of the reasons why iPad sales are floundering is that Apple isn't giving users much of a reason to upgrade. And Apple can't do much to force users to upgrade for a few years.
I believe the reason for weak is quite simple - iOS 9 supports every iPad Apple has made except for the first-generation iPad. That means everything from the iPad 2 onwards is currently being supported. And remember that the iPad 2 was first released back in March 2011.
Now it's true that the iPad 2 doesn't support all the bells and whistles that iOS 9 has to offer, but I'd be willing to bet a steak dinner that a randomly selected Joe Average iPad user couldn't name a single feature they're actually missing out on. As far as they're concerned their iPad works and is still receiving updates.
And the problem is that Apple didn't discontinue sales of the iPad 2 until March 2014 (the iPad 3 was discontinued in October 2012, with the iPad 4 being discontinued in October 2014), which means that the iPad 2 owners will almost certainly be offered iOS 10 too, which means iPad 2 owners will be able to hang onto their tablets for another couple of years.
By offering the iPad 2 for as long as it did - it was offered as the cheaper alternative to the iPad 3, iPad 4, and the iPad Air - Apple has steered itself into a position where it has to support a product first sold in March 2011 until at least early 2017. Maybe this was deliberate, or maybe Apple thought people would be more eager to upgrade, but either way this isn't helping Apple shift iPads in the now.
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What this does mean however is that come 2017 there will be a whole raft of iPads that are no longer getting iOS updates drip fed to them, and it'll be interesting to see if the owners of these aging tablets will feel like shelling out for an upgrade.
Now you might be wondering why the iPhone isn't suffering in the same way. After all iOS 9 supports the iPhone 4s, and that was only released a few months after the iPad 2. Bottom line it seems the difference is down to there being a different upgrade cycle for smartphones than for tablets. People upgrade their smartphones every two to three years, but hold onto their tablets for longer.