I see that too in my area, as one commenter said, private sales/trades usually are more profitable for getting the newest phone.

Now in the sense of the iOS updates, I don't see any difference with my battery life or battery health, mind you I'm new to iPhone (since black Friday weekend 2022) and I have the 13, so I basically went through iOS 16.1.2, 16.2 and now on 16.3, so I haven't seen many issues, I think what people don't realize is that with whatever improvements to the iOS that are done, it will take a toll on the battery for the first day or so for the phone to recalibrate