Statistics indeed show that 3-4 years is the typical time people switch their phones at, but I don’t think your reasoning is sound. You can cheaply replace the battery at any number of repair shops.

It’s simply that 3 small, incremental upgrade does make up for a more significant one in the end, though in the future it might end up 4 or 5 years instead, as the changes from generation to generation are smaller and smaller.