Same as with cars
This goes to show that "use until it is dead" model is significantly better financially than "change shiny-shiny every 2 years". The fact that it sucks marginally less for an iPhone is just that - "sucks marginally less".
So if you run the same model till full depreciation which for a phone now stands at around 4 years you get that you have a net gain compared to selling old and getting new every 2 years.
That is not news - it is well known from other goods - cars, white goods, etc. Just there the depreciation periods are longer (up to a decade).