There hasn’t been enough development and must have features to really compel me to upgrade.
I was still using my iPhone 5 (16 GB) until this last January (Which I purchased new).
I felt it did everything I wanted it to do. And still more than I needed.
My iPod Classic (160 GB) was stolen in December. I missed my huge music library in the car. And a new iPod of similar capacity put me in the same price range as a new phone.
So I purchased a used iPhone 7 with 256 GB of RAM. Solely upgrading for the storage capacity that my iPhone 5 lacked.
Honestly, if it hadn’t been for my iPod being stolen, I’d still be happy with my iPhone 5.
Having been using the iPhone 7 since January, I still don’t feel it adds any significant value or function over my iPhone 5. Other than capacity, it serves the same needs and not really significantly better.
Granted, Apple abandoned (blocked) continued 32-BIT app development for my old iPhone 5. But in truth, by that point, I really actually had all the apps I needed. And they still did everything I needed. And, actually, my favorite app hadn’t been updated to 64-BIT, so I had to settle for a significantly inferior replacement app when I upgraded.
From where I am now, I have no desire for an iPhone 8 (hardly any significant change from the 7). The iPhone X, is honestly a complete repulsion for me (I prefer the home button method - and have actually grown to like the fingerprint method).
So I guess I have adopted the fingerprint since updating to the 7. It’s useful. I wouldn’t have updated to get it, but I like it. I absolutely do not like the face recognition concept. It’s got a lot of potential weaknesses in concept if it became a problem of privacy with people who have access to you. And likewise with people who are nearly identical in appearance to you (some families have practically twin like resemblance - like mine)