Even a bigger battery is going to degrade at the same rate. It might limit the need for CPU throttling, but probably not by much.

Seeing as iPads are basically iPhones with much larger batteries, and don't suffer from the same issues, I disagree.

This could have been avoided if they weren't so obsessed with thinness. I reckon that if it was thick enough to not have a camera protrusion, it would be the perfect balance of form and battery life.

A larger battery, running the same CPU, with the same stated "battery life" as the original battery, will degrade gracefully for a longer period of time before the compute performance must also degrade.