I think they were in the luxury position to try bumping up the prices a few generations; up until and including the X that strategy was successful, but now people are voting "no more". Or well, it's still selling, just not as much that it's causing Apple's revenue to keep going upwards. Apple is still one of the most successful companies in the world so I wouldn't call it a problem / failure just yet.
Yep. As I written in another sibling comment, there is a ceiling to what people would pay even if they love the idea of the device. And Apple has hit that ceiling.
I know people who can easily buy 10x iPhones for their entire family and part of their relatives -- and they refuse. These are people who can spend $50k and almost wouldn't notice and even they think the modern iPhones have taken the price too far.
