Also replacement batteries for old phones will probably cost nearly as much as new phones and will have a huge profit margin for phone manufacturers.

why? it will be fine.

mildly overpriced batteries of varying reliability will be ubiquitous. you will be able to get rid of that warning message with a stop at a gas station, or perhaps…

a low monthly fee.

perhaps rather like toothbrush head, you could get a new one periodically. on subscription. just throw the old one in the trash, and it will magically disappear to somewhere else.