They want a bigger screen because they don't have a laptop.

> save the money they would have spent replacing it (plus the money they would have paid extra because larger is more expensive) to buy a cheap laptop or something

The kind of people who want an iPhone are not going to settle for a Cheap Laptop. A MacBook Air can only really be had new for around $800 nowadays and those big iPhones are only like $599 right now (if iPhone 16e).