It's also because the smartphone became mature. Many of the earlier iPhones had gigantic deltas: 3G, Retina + front-facing camera, Touch ID/secure enclave, > 4

It's also because the smartphone became mature. Many of the earlier iPhones had gigantic deltas: 3G, Retina + front-facing camera, Touch ID/secure enclave, > 4 inch display. Now (or at least for now) the changes have become more iterative. The delta between several generations are still nice, but between individual generations are less enticing to immediately upgrade. In the end this is not bad. Upgrading a still perfectly working product every 1 or 2 years is quite insane and taxing for the environment. A lot of credit should go to Apple for maintaining iOS support for old iPhones. This makes it possible for people to keep their phones longer or hand them down. Or for people with lower incomes to buy older generations and still get upgrades. Of course, slower upgrade cycles are bad for manufacturers. So it's smart of Apple to increase revenue through services (Apple Music, Arcade, Apple TV+, iCloud Storage), rather than forcing their customers to upgrade perfectly-fine devices.