Here's my hot take: Apple knows more than we do about pricing and market realities. The new iPhones are more expensive because Apple knows it will sell fewer and fewer iPhone upgrades (due to the maturity of the technology and high customer satisfaction with their current devices). To cope with falling sales numbers, Apple is raising prices on the fewer phones it sells. As sales volume goes down, the unit price goes up. In terms of actual price increases, I don't think most consumers are bothered by a few extra dollars a month on their carrier bill due to a slightly larger phone payment. The problem is that consumers don't see the point of upgrading at all.
TL;DR: iPhone sales aren't dropping because the price is rising. Prices are rising because Apple knows they've reached the point where upgrades (and therefore sales volume) is decreasing, and are raising unit prices to compensate.