I think it's mature in the sense that future growth from the segment is limited, at least compared to smartphones. Short of lowering the price, I don't see much that Apple can really do to accelerate growth. The iPad Air 2 is an amazing piece of technology, yet all it really offers is a slight performance boost over the original iPad Air. Plus we all know Apple will never compromise on the price of iPad in order to increase sales.
I agree with you about smartphones. That's why I think the 6S next year will again sell more than the previous iPhone. Smartphones have become essential tools whereas tablets have not. We use them all day everyday for the most part. The financing for phones isn't really a knock against the American consumer since from what I've seen the carriers allow it with 0% APR. That makes it pretty fair in my opinion. $25/mo at 0% APR is a steal. I personally told myself I'd keep my 5S and skip the 6 but when I saw the deals the carriers offered, I bought a used iPhone 4 on Craigslist for $50, traded it to my carrier for $200 and got my iPhone 6. It just made sense.
Apple's strategy of keeping the same form factor for 2 years is really ingenious and no other manufacturer is able to get away with it. I'm willing to bet they will be far more efficient in manufacturing the 6S next year and will be able to more easily meet demand, in turn allowing them to sell even more than they sold this year.
What limits tablet use right now is use cases, if they'd be seen as something other than mere media consumers, they'd get replaced more often. That's why Apple will tightly integrate them with the rest of the devices and ecosystem and overpower them. Eventually something fantastic will come to the tablets (or the rest of the ecosystem) and everybody will get one.
As for cost, considering that for most people (not power users) a tablet will soon replace a desktop, laptop, netbook, portable game console, even camera, the price is ridiculously low. People were paying more than twice as much for a C64 in 1983 with 1/100 the usefulness.
If something is useful enough, people will switch more often, or in the case of Apple, drive the sale of other Apple devices. Apple wins if every Apple device in the home is a disincentive to switch to another brand. That's their whole strategy.