I only got an iPhone because I was going to do some app development, but eventually lost interest, partially because Xcode and the byzantine Apple certificate process made me hate life.
I, personally, think smart phones peaked with the Palm Pre and WebOS.
I honestly can't think of a feature I've seen in the last 5-8 years that has impressed me at all, maybe LTE?... the only things that have improved mildly are some specific apps and none of them seem like they should even need faster CPU or more RAM if OS updates included optimizations rather than bloat.
But, perpetual growth economy! So you better believe that if they can't innovate a new killer feature to convince people to upgrade they'll be "innovating" a new killer usability degradation to do it instead.