So I guess newer iPhones and iPad allow you charge up to 80%, which extends battery life, for idiosyncratic reasons? I’m sure there must have been a reason and demand for that.

I guess I run my iPhone on low battery mode a lot, due to idiosyncratic reasons too. Maybe I do.

Apple battery replacement costs anywhere from $70 (for a ~$400 phone) to $120 (for a ~$1000+ phone). In many global markets you can get a brand new phone for that much.