Don´t be naive.
This is a phantasy-calculation.
Your provider is better than you:
He has to pay not much less than you would pay for it - even for them, iPhones do NOT fall from heaven - so he calculates HIS "losses" within your contract.
Contracts WITHOUT inclusion of new Smartphones every year are much less expensive - even if you get a more-or-less "good" price for the "old" one..
There are many statistics showing that even buying a phone AND signing in a cellphone-contract separately from each other is less expensive - and so you are even free to buy also a "second hand" top-phone instead of a "new" one just at that time you need really a new one.
So - if you talk about "phantastic" resales of yearly "donated" Smartphones of your Cell-Phone-provider you have to calculate all together.
And I can say: ALWAYS the bottom-line all-together is: That type of "all-inclusive"-Contracts makes you even lose more money than a "Communication only"-Contract without yearly gadget and a free buy of a Smartphone from time to time (i.e every 2-4 years) … there was not much REAL "innovation" since iP5…. and at tat time banding it was impossible …
edit:
"Resale-losses" are NEVER about a certain percentage of price when purchased , they are about bucks in real numbers.
Will say:
50% of 1000 USD = 500 USD lost.
50% of 800 USD = 400 USD lost.
And even with a product with more percentage of "Losses" (= poorer resales at first look) , lets say even 60%, will let you loose less than the ones with "good" resales - if you have to pay less when purchasing the new one with "less chance for a good resale"….
Never calculate in %, calculate always in USD…you will be surprised sometimes...