There are a lot of differences (improved camera, edge-to-edge display, Face ID, much better SoC). However, SoCs and cameras are already so good that the average

There are a lot of differences (improved camera, edge-to-edge display, Face ID, much better SoC). However, SoCs and cameras are already so good that the average consumer does not notice the gaps as they did with e.g iPhone 3G -> iPhone 5. This will be a term problem for many phone manufacturers, the market does not grow as fast anymore + people keep their phones for more cycles. Anecdotically, many of my friends and family members would upgrade their iPhones every two years. Now most of them are still using the 6s, SE or even 5s.