First time I hear about this upgrade program and it's rough, for the $999 Pro it comes out to $600 per year, every year, as long as you want to keep doing the yearly upgrade to the latest iteration. And when you eventually decide not to upgrade anymore after the announcement of a new model and you decide to let the upgrade program run out you still owe another $600.
Let's say you started with the 12 Pro, got the 13 Pro, 14 Pro and now the 15 Pro and then you stop upgrading. The total cost is $3000. If instead you just bought a 12 Pro and traded it in for a 15 Pro now the cost would be nearly halved, $1638. If you wanted the equivalent AC+ coverage it would still be considerably less, $2238.
And after all is said and done you'd still be the owner of the same iPhone 15 Pro. Is it really worth almost twice the cost to replace it every single year? And that's before looking at actual resale value and carrier deals.
The program doesn't even make sense for the cheaper non-Pro iPhones as you're paying to always be on the latest tech and the regular iPhones trail a year behind the Pros. It would make more sense then to pay a bit more to get the Pro model and replace that every two years.