I understand the sentiment here. At a high-level, this can be done "reasonably" where the actual "cost" of upgrading every year isn't as big one might think originally, largely because of the way the secondary market has dictated the depreciation curve of iPhones.
After 1 year, they don't depreciate "that much". After 2 years they drop off more, and after 3 years, more so.
Total round numbers and completely based on gut feeling, I think it's something like:
- After 1 year, 80% value
- After 2 years, ~50% value
- After 3 years, ~30% value
There's lots of things that can move the value one way or the other that many ppl have brought up (ie. higher-storage capacity models probably depreciate a bit more on a % basis, friction costs in selling on Ebay/Swappa/FB vary based on location and experience, etc.)
But I don't doubt that if you're on the happy end of a lot of those "little variables", the actual cost to upgrade every year may not be that much more than if you wait every 2 years or every 3 years to upgrade. And all-the-while, you get the benefit of having the latest phone every year.
I personally don't do this, but I can see why it may make sense for some people. The decision makes more sense if you have more disposable income (so the $ cost isn't as big a deal) and you really enjoy having the latest tech (so get more "value" out of upgrading every year and always having the newest).