The typical lifespan of an iPhone can easily stretch to 10 years after its launch. While Apple officially provides iOS updates for about 6–7 years, that doesn’t make the phone unusable once support ends. In fact, most iPhones continue working perfectly well even after updates stop. You still keep getting security updates.
The main limitation comes 3–4 years after iOS support ends, when many apps begin requiring newer OS versions. At that point, upgrading may become more of a necessity, but the device itself still functions just fine. I have 6s to back this up.
Apple’s current development tools (Xcode) continue to support development of apps running on iOS 15, which means devices like the 6s haven’t been fully left behind yet.
I firmly believe you don’t need to upgrade your iPhone unless it physically fails. A simple battery replacement is usually enough to keep it going until the very end.
One edge case happened with our UPI system, which unnecessarily demanded a higher iOS version.