Earlier quoted context omitted.
Device design is always constrained by the current technology. It isn't impossible to make a phone with current battery tech that doesn't overheat after a year or two of normal use Apple just pushed design to far and underestimated the cooling/heat dissipation required
Heat had nothing to do with it. The internal resistance of the battery increases as it wears out, lowering the peak output current. Increased heat when operating near the current limit is a symptom, not a cause. Adding a fan or a chonky heatsink to your iPhone wouldn't magically raise this limit.
A two year old lithium battery under normal use will hardly degrade at all. Any design that pushed the limits so far that a degredation of a few percent over promised and under delivered. In Apples case it could be remedied with a software update, but that doesn't mean the device held up to the original performance claims over a standard life cycle of device use.