There are a lot of posts with reasons for Apple's poor sales. Some people claiming that the iPhone is too expensive, while others claiming that it isn't a pricing issue, but that there isn't a need to upgrade to a new iPhone when the consumer's current iPhone does everything they need to do.
I bet the truth is probably somewhere in the middle.
Maybe iPhone sales would increase if the prices were lower, and people that wouldn't normally upgrade due to being happy with their current iPhone, might consider it due to the lower prices.
Also...
Maybe iPhone sales would increase if Apple would introduce some new innovative features that are actually useful, and people that wouldn't normally upgrade due to the price, might do so to have these new useful features.
I think Apple was once really good at finding a balance of the two things above, but with dropping the SE and increasing the price of their flagship model iPhone, while leaving the utility of it mostly unchanged over the past 3-4 years, people are losing reasons and incentives to upgrade.