Comment by Retric

You can replace the batteries at both Apple stores and independent retailers. https://www.cnet.com/how-to/iphone-battery-replacement-diy-o...

The issue IMO, is it's not obvious that the battery needs to be replaced and when. It's not holding charge > new battery is far more obvious than it's slow > new battery. Most people probably prefer a slow phone than one without charge however Apple should have some indication of what's going on.