It doesn't matter, second hand or not every day devices like phones do not last for say 10 years on average even iPhones.

Apple only needs a device to last 6 years(on average) to keep current sales levels, that includes all the irreparably damaged, or irrecoverably lost and bricked ones after theft etc.

Even if a device did last 10 years on average, Apple will still sell 183M units a year, iPhone is only 50% of the total revenues, so roughly 5% of their total revenue would be impacted, that is also ignoring additional revenue generated due to service and parts and batteries needed for older phones between 6-10 years.

A net impact of say 3-4% of revenue while significant wouldn't really change their fundamental numbers or long term market value. The market has already accounted for no further iPhone sales growth in the models anyway for last few years.

They need to only convince people over the next few years on the value of Apple Intelligence not with 16 right way to upgrade, coupled with lightening to USB switch since 15, it will be enough of motivator for 11 to 14 users to switch during the 17/18 cycle and beyond.