Seems like if that was Apple's goal, it would be easier for them to just reduce the quoted battery life of the phone, limit the charge to 80% for everyone, and relabel the 0-80 scale to 0-100.

Presumably, they've already effectively done this by choosing voltage amounts that balance warranty repairs with battery life. e.g. the iPhone 15 charges to a max of 4.48V, but the battery presumably physically supports some higher voltage for "100%" that would give more capacity and less lifespan. (Notably, different iPhone models have different voltage ranges.)

"reduce the quoted battery life of the phone" - This is the bit that presumably they don't want to do.

Reducing the top-line specs of the phone means that it might fall down in comparisons to others in a spec-comparison.

I presume they would rather move the conversation into customer-land, i.e. once you've bought your phone they talk about how they are acting to preserve your expensive device.