Good point, but that still supports my overall point; you’ll need higher tolerances to prevent shorts and fires which means you need thicker casings. A user-replaceable battery has thicker battery cases and connectors compared to devices where the battery isn’t accessible.
Look at laptops for a similar story; making batteries user-inaccessible allowed them to shed thicker casings and instead fill more space when they weren’t constrained by a user compartment and casing and need for easy-detachable connector. Going back to a user-removable design in the exact same size case means slightly lower capacity batteries, which customers don’t want as a trade off.