The problem with that hypothesis is that a non-trivial number of iPhone customers (that I anecdotely know) tend to upgrade every few years, not every iteration.

The problem with that hypothesis is that a non-trivial number of iPhone customers (that I anecdotely know) tend to upgrade every few years, not every iteration. If a device is "good enough" to last 2+ years, I'd say Apple is doing a decent enough job at keeping their users happy.