Even with Apple earbuds battery could be easily replaceable if the cylindrical part of the piece was screw-able to the top. Just like a flashlight.
Bedsides, how many AirPods are likely to be dying from poor battery life compared to being lost, damaged or upgraded?
If this did happen, I suspect there would be more instances of people losing the battery cap, or throwing it away because of undiagnosed battery terminal faults, or of children dying because they swallowed the impossibly small pill shaped battery...