And why shouldn't it drop?
We've already got iPads we're happy with. These aren't smartphones. There's no need to buy a new iPad every year.
Once we all have these devices with long shelf lives, sales will level off.
In the meantime what has Apple done?
They released the iPhone 6+. Less of a reason to get an iPad (though there are still reasons to get one), and more of a reason to just get a bigger phone with a great display. It's just a smaller iPad but...