My current iPhone's battery capacity is already starting to decrease and it was never great to begin with (needed it for work). If it was replaceable, I'd do what I used to with Android phones years ago - get a spare, if the old one is really bad or turning into a pillow, then recycle that and keep using the replacement, otherwise could use both side by side and didn't even need a separate charging bank.
Lots of people will look in the direction of getting a new phone altogether, I might have to do that as well, turning the whole phone into e-waste, instead of giving it 5 more years of lifetime.