Depends on features. Using a 6s at the moment and phone is still fine. Don't feel I need the latest and greatest, bit tend to upgrade based on features and when mine starts to slow down.
Think 90% of people buy under a contract so to some degree cost doesn't matter. Apple upgrade has only been out since iPhone 7 in the UK, but one of our operators, O2, basically does the same, giving you a airtime plan and a phone plan rather than the older combined plan, so when the phone is paid off, bill automatically gets cheaper, also can pay off anyone to get out of contract - which is how I usually buy my phone.