It's more tied to the change of the business model.

Phones used to be subsided by the plan and the 2 year contract lock in, so if you didn't upgrade every cycle you were effectively leaving money on the table.

This is why the market accelerated so quickly compared to any other class of hardware.

As the 2 year contract fell out of favor (thanks largely to T-Mobile), you had 2 year heavily discounted payment plans tied to device trade in that took their place, but these were opt-in as opposed to the previous model which was built in to every contract.

While the economy was strong, the depreciation on your current device and effectively FOMO on maximizing its trade in value kept the system driven at similar numbers.

But as purses have tightened, suddenly the outlay on increasingly expensive devices with lower trade in values for past devices is a racket people are opting out of.

It was never really about features as opposed to status and reup indicators. Most of the rest of the world has more like 3 year phone replacement cycles for the past decade, and have been fine with the same feature parity per model year as US phones.

I'm honestly surprised the 2 year thing kept going as long as it did.