Actually, reading your comment made me realize that perhaps one thing that is making people less likely to upgrade is that there are now so many versions and models that it's not readily visible how "old" your phone is, so there is less status-climbing pressure to upgrade.
I mean, when I read your comment, I couldn't remember how old the iPhone 7 actually was - I had to Google it. The iPhone XR, XS Max, XS, X, 8 Plus, 8, 7 Plus and 7 were all released in a span of just over 2 years. With so many different models, I think there is probably less social pressure to upgrade because it's no longer like "OMG, why do you have the 3-year old iPhone?!" when no one can remember when all the jumble of versions came out anyway.
After the 7 is the iPhone 8 and the iPhone X. And now there is the iPhone XR and iPhone XS. So there are quite a few "better" phones above the 7.
My wife still has has 6S and it's more the capable. But if you were going to buy and "new" iPhone today the perception is that the 7 is actually quite "old".
The other commenter makes a great point about the sheer number of phones released in a short time span.
