If you can open up your watch with a coin then obviously the design is affected by this being something you can do. Saying that it doesn't interfere with design

If you can open up your watch with a coin then obviously the design is affected by this being something you can do. Saying that it doesn't interfere with design, size, weight, or price is disingenuous. Of course it does. It might influence those things positively or negatively, but it absolutely is affected. If you have to put in a mechanism to easily remove a battery from a phone then how wouldn't that affect literally all of those factors? What sort of properties would this entail and what would be suitably easy to remove? That answer is different depending on the person. Some people perhaps just want the back to slide off, which would probably require a plastic back. You could add a hinged door, but that would certainly add to its thickness and affect the design. You'd also need to have a way to interface the battery to the phone. Phones that had easily replaceable batteries had contacts built into the phone and the battery that were sufficiently large as to make it nearly impossible for someone to fuck up the installation of a new one. This obviously added weight and thickness to the phone. I'd love to hear how you'd engineer it to be just like a phone without an easily removable battery without affecting the criteria you listed.