I think it is not only that the older iPhones are so great that fewer people are upgrading, but the newer iPhones are not that much better than the older ones that it just isn't worth upgrading.
I am not disagreeing, but raising of the unit price also leads to people holding off to upgrade too.
If someone was on the fence about whether to upgrade or not, the increased price of the new iPhone could make the difference and lead to people choosing to stick with their old iPhone.
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.
I am not disagreeing, but raising of the unit price also leads to people holding off to upgrade too.
If someone was on the fence about whether to upgrade or not, the increased price of the new iPhone could make the difference and lead to people choosing to stick with their old iPhone.