The thing is, tweaked iterative versions are not that appealing to the consumers.
Says who? Look at the iPhone 4S... Probably the best selling phone known to man.
Regarding margins... It was inevitable due to OEMS flooding the market with cheaper phones and tablets but looking at it in a one dimensional manner serves no real purpose.
Apple's margins are constricting due to a number of factors.
1. Older phones are still selling well, especially in developing economies.
2. New product lines like the iPad mini which came with new production processes and increased costs. This affected iPhone 5 margins too. (I fully expect to see margins rise a few percentage points again due to product stability in the future). Historically Apple's margins peak during the S release cycle.
3. Even though no new products have been announced for some time, R&D expenses have remained stable.
All these contribute to the constricting margins.
I'm also of the opinion that Apple will drop the 3.5" inch screens when they release the cheaper iPhone. This cheaper product again has the ability to drive margins upwards in the future if priced correctly due to the cheaper component costs and 1 generation behind in breakthrough tech.
I've been toying around with the notion that, due to the cheaper iPhone, Apple will cease to supply older models in the future to drive margins. So what we will have is only 2 iPhones on sale at any given time ie the premium iPhone and the cheaper iPhone with the previous releases EOL'ing.