I think the reason has nothing to do with the iPhone 8, 8 Plus, X, XR or XS. I think the reason has to do with 2 back-to-back iOS updates that didn't hugely slow down my iPhone 7 Plus.
I'm used to really wanting the newer iPhone when I update iOS 12 months after buying but the last 2 updates haven't fit that mold.
There is also just a general kind of burn out. In phones, in social media, etc. Maginal utility from consumer tech is now so marginal that people are starting to not upgrade every year. It was bound to happen. I’m sure someone will come up with a more creative name for this when it starts to impact stock prices.
iPhones and the iOS speed focused updates are really showing the longevity of the device. My wife has the 7+ she got when it came out. I got her the battery change out for $30, and the device is great. It got faster with iOS 12. She has no need for a better device. Upgrades now really are incremental, with much of the technology 'good enough'.
I have the X. It works great. Nothing in the XS makes me want to upgrade. I would like the XS Max, but I'll just wait for next year. And next year, my wife will get my X which further extends its usefulness.
I'm used to really wanting the newer iPhone when I update iOS 12 months after buying but the last 2 updates haven't fit that mold.