People can and do replace iPhone batteries, but it's usually taken as a sign that you had the phone long enough to morally justify getting an update. The old phone with dead battery is very often sold and the battery gets replaced - Apple sells refurbished phones too.
It was more common when they were user serviceable
Do you seriously think many people are not trading in their super expensive phones? I don't know anyone who doesn't...
I think this was more true 5 years ago. Most people I know just replace the battery because it’s 10% of the cost.