Several factors at work with these slowdowns.
iGizmo market saturation, commoditization of component prices, mediocre,
same old look updates, competing products that offer more features for less money
and last, but not least, the law of large numbers.
Existing markets have changed, their numbers reflect it. The 2 year upgrade pricing model has disappeared, consumers have to pay full price for their devices. Emerging markets should compensate, they're not there yet.
The price increase caught me by surprise too. Purchased the iPad Air 2 for 510€, now they want to charge me 869€ and force me to upgrade from 64 to 132 GB? Simple answer: No.
If I were Apple, this is what I'd do from most important to least.
(1) Revamp the Apple Watch entirely. Call it the "iWatch" and do something like what Garmin has done with their VivoActive HR. Bring the product back to "fun" instead of "fashion" like the iPod was back in 2002.
(2) Stop going for this "thinner" crap. Make the iPhone battery last insanely longer AND give us wireless charging.
(3) Make Siri work. Fix it. It's ****. Come out with a similar product like the Amazon Echo.
(3) Make the iOS UI a lot smoother and follow Apple historically phenomenal design principals.
(4) Improve Safari ... its old and ... just work on it.
(5) Improve iTunes ... it's ****. Plain and simple. Break it up if you need to.
(6) Make Messages app universal like iTunes.
(7) Beef up the computer lines: (a) a mid level upgradable computer (b) more beefy laptop specs.