Gurman lists various reasons, but I feel like the biggest one by far is that the pace of smartphone development was incredibly exciting from 2007 to like 2017 (the iPhone X). Maybe a little later, but at some point you've got a fantastic camera and a very fast device with a great screen, so why would a normal person upgrade?
I think the cheap iPhone is one valid path, but you could also take the completely opposite direction, go all-in with on-device AI, with tons of RAM, a massively upgraded Neural Engine, etc. Find another big leap forward to offer.