Because of how aged batteries work, if you don't purposely slow down the phone sometimes it'll shut off, which is worse.
Maybe Apple should use their proverbial ingenuity and inventiveness to invent a smartphone design that lets you open its back shell without screws to allow the user to replace their own battery, though I'm not sure if the technology for it is quite there yet.
They're not even the only consumable part anyway.
>Batteries ... are already easily replaceable
Is this[1] what easy looks like?
[1] https://www.youtube.com/watch?v=T0fUmW-2swg