Wait, do people actually upgrade yearly? I always thought it was hyperbolic

It's not common (I think average upgrade cycle on iPhone is a little more than 3 1/2 years now, and getting longer every cycle) but it's not unheard of. The iPhone hold value well enough that you can often trade it in at something like a $200 discount and just get the new one.

Considering the cost of a battery replacement, it's not the worst value proposition in the world.