Even though they have more cash stashed than most nations, I am starting to think more and more it's an issue of scale.
What I mean is that even if they think up a radically new case, they need to manufacture an obscenely large quantity now (they sell WAY more phones than they did when the iPhone 4 came out - don't forget, you couldn't even get an iPhone on Verizon until like a year after the 4 came out). On top of this, they still rely on 3rd party manufacturers - and THEY need to be able to handle those quantities too (at a very quick pace).
Simply because of their success, I'm wondering if we see longer cycles of "lipstick" on core design, while they can iron out the bigger changes. What used to be something they could plan and produce with 2 years lead-time has probably ballooned into 4 now.
Sometimes, being smaller and more nimble allows you to be a bit more radical.
I fundamentally disagree with Dustin's conclusion. Long-lasting and "as little design as possible" are two cornerstone principles of Dieter Rams and his contemporaries. As manufacturing and electronic technologies improve and "catch up" to the designer's original intent, those will drive Apple's industrial design as their flagship products mature.
To label that design approach as "stagnation"seems shallow and short-sighted, and ignores newer products such as AirPods (which I'm sure involved a level of design work most people take for granted because of Apple's high standards).