Wow Apple earns 50% of your phone every single year now if you keep trade in. Who said keep trading would not cost more than just holding existing iphone? So crazy expensive to trade in every year now.
50% after 1 year is not bad considering you don't need to do anything. Selling it on your own might net you like 60-70% but it's more of a headache and some risk involved.
2 years is really the sweet spot to upgrade in my opinion. That's assuming you sell the phone yourself. You'll generally lose $200 a year in depreciation whether you keep the phone 2 years or 4 years.