Those screw in backs add a few millimetres of thickness to the watch. They also need to be made from thick heavy materials like stainless steel. When you make them out of lighter materials, like plastic, they are far thicker.

It's not rocket science, but it is a challenging problem and one that can only be solved with compromises. Personally I'm fine with those compromises, but you're kidding yourself if you think anyone is going to buy a smartphone with a 4mm thick steel battery cover.

What's probably going to happen is phones will have a screw on cover with a weak adheisve that will be destroyed when you replace the battery. You'll need to remove the adhesive and apply new adhesive if you want to maintain water resistance.

Also - watches aren't compliant are they? You can't take the back off a watch with a screwdriver. Dive watches would fall under the "designed to be submerged" exemption, but regular watches willl probably ahve to switch to being held in with screws instead of a screw in back - and a lot of currently waterproof watches will become water resistant.