Just like how it already work with watches. You can replace batteries yourself, but waterproofing is not guaranteed. If you want to be certain that the waterproof rating is maintained, you have to sent it to an authorised service provider.

Yeah great

“let’s just pass a law that says ‘no trade offs’, problem solved”

You will need to have a factory to replace the battery and also run tests to prove that the phone still works with water then

"I would rather have IP67 over a replaceable battery"

Not me, and not most people.