Some people value having the latest phone, latest cameras (legitimate reason to upgrade imo), latest specs outside of speed and processing power
Most people on this site are phone enthusiasts and upgrading annually or even every two years is normal for us. Crave that new design or every iterative upgrade, big or small. Will sell the old phone to pay for the new one or justify that annual fee/contract renewal to upgrade. I'm more in this category myself haha. (You're probably not in this category if you think its ridiculous to spend all that $$$ year after year on a phone despite recycling the old phone towards this new one)
Other people are motivated by speed differences...not "15% faster" but "my old one is so slow now its painful to send texts"
We can see why this group might have a hard time justifying the newer iPhones over their old ones...one of my best friends who is a super whiz with computers and tech geek continues to use his 6s and absolutely loves it. It's plenty fast for him - he's right.
4 generations apart yet almost identical in speed for a typical user's daily use.
Compare these to the previous 4 generations (tough to find a 6s vs 5 comparison but note the A6 vs A9 with the 5 vs SE):
The speed difference from A9 -> A12 is
much less than what that 4 generation gap used to be. I could show an iPhone 4 vs 5s video too but that is even more stark. Apple is so good at these chips that they will start lasting longer than ever. They promised as much when the A7 was announced. For those people, their 6s or 7 is perfectly fine and useable today and im sure we know a few of those people
Others yet simply purchase an item and use it till it stops working. My mom and both my sisters fall into this category. They could care less about their phones having speed and cameras.
Some of us on this site fall into one of those three categories or we know someone who does. Most of us seem to be in the first two, less of us in the third. Nothing wrong with any of them. We can all debate and discuss and say which way is best or when to upgrade but the truth is, if someone wants to upgrade they will. If someone wants to keep their older phone, they will.
(PS i am sure there are other categories we could create, but i would argue they all overlap with one of these 3 in some regard...upgrade every year or two, upgrade when the phone they use becomes unbearably slow for them, or use it until it stops working - no matter if it gets slow or not)
(PPS I also think that whether or not one can afford it, plays less a role than one may think....I know people who could easily afford every new colour of every new iPhone and every new Apple product that comes out with each upgrade without thought, but fall into the "use it till it breaks" category. One of those individuals who comes to mind is still using a 2010 or 2011 MBP, and was using an iPhone 4 until it broke and reluctantly upgraded to a 7 right as the 8/X were around the corner)