increase cost

Probably, but that’s why you make trade-offs in the design process. With a BOM of $236 and a MSRP of $750, it’s not like we’re starting from a razor-thin margin here.

and probably also the size of the phone

Doubtful - total charge capacity does depend on volume, but nominal discharge current is mostly orthogonal.

We don’t expect car manufacturers to produce engines that never need oil changes.

Of course not. But if a car manufacturer designed their transmission so that after 40,000 miles, gear 7 and 8 stopped working because the gear teeth were too worn down by that point, and the automatic selector would top out at gear 6 to avoid stalling the engine, you’d definitely have a lawsuit and recall on your hands.

This is all about norms of consumer expectations. People expect that battery life of their devices will degrade over time. They don’t expect that performance will do the same.