Phones are good enough that they feature sets last 6 years, while the batteries don’t. Plenty of people don’t want to drop $800 on a new iPhone every few years

Phones are good enough that they feature sets last 6 years, while the batteries don’t. Plenty of people don’t want to drop $800 on a new iPhone every few years when their old one works fine. And those who do move their old iPhone to the secondary market. The secondary market exists because contrary to your statement, plenty of people are fine with old phones.