A 2 year old iPhone can still be traded-in at roughly 50% of the value (whether to Apple or a third-party). The effect that this creates is that it becomes quite reasonable to buy the latest iPhone every two years after trade-in. I'm sure this has a big impact on new device sales, one of the factors as to why Apple beats records every year.
On the flip-side, I believe the reason they are able to do this, as the article here notes, is that their phones just last long (hardware/software). My iPhone 8 is not very old, but I'm not planning to stop using it for another 2-3 years at least (unless it shatters). In some ways, this reminds me of ThinkPads, just in terms of how long they can serve their purpose.
I'm honestly getting kind of "bored" of Apple devices, but I fear Android devices may not have the same solid 5 - 7 years of support, and as is evident from this article, trade-in value.
[1] https://www.bankmycell.com/blog/cell-phone-depreciation-repo...