Used to have an iPhone 6, which I bought a 64 GB model for $299 under contract before AT&T ended contracts and my phone had a grandfathered unlimited plan. I sold it for $140 last year since the battery is starting to go and I bought a used unlocked iPhone 8 64GB with a Qualcomm modem. The iPhone 8 is still plenty fast and the iPhone X and newer phones are way too expensive and iPhone XR is inferior. I don't see myself upgrading anytime soon, maybe until 2022.
Mobile wise, my iPad Pro 9.7 and newly bought iPad Pro 11 get more use than the iPhone 8 these days and I feel that the iPads give better value in running productivity and iOS apps than the iPhones which cost a lot to get storage above 64GB. Getting higher capacity iPads are cheaper than iPhones. I tried Face ID on the iPad Pro 11 and it's convenient, but I can't justify spending more than $1000+ to upgrade from an iPhone 8, while it still cover my needs.