Usually the battery fails long before iOS updates stop, or the the hardware becomes unusably slow. And many folks don't want to deal with the battery replacement headache, so they just take their 5-7 year old iPhone to the store for a trade-in/new one.I have friends that still use iPhone 5s, and are very happy. I played with one of their phones during a visit, and they still have really impressive speed. They make calls, send texts, surf the web, etc. with grace. Certainly they don’t have some of the more modern and “hip” Apple features - like “memogies” - but they’re practical and strong. Apple hardware has always been superior and these old phones don’t just die when iOS updates stop coming down.
Seems like a reasonable timeline to me, and independent of iOS updates.