Cosmetic changes like different colors and materials make it possible to distinguish who has the latest model and who has last year's as well as between the base and the pro models. It may seem pretty subtle, but status signaling is often subtle at the high end.
I'm not trying to argue that most people upgrade yearly. I don't have any knowledge or opinion about that. I'm also not saying people only buy iphones for status. I have one myself, for pragmatic reasons. I'm just pointing out that a majority of people owning a particular brand doesn't mean people can't signal status with that brand. They can still do it by owning particular models. You see this all the time with cars. The a new high-end Lexus conveys status even though you see a lot of old beat up Lexuses being driven around by poorer people. The subtle design changes of iphones from year to year, I would argue, are meant to provide that same effect.
