Replacing batteries in waterproof Phones isn't a great challenge by an tech service. Repairable don't necessarly mean DIY for the user. Most repair in phones need anyway an tech service. iPhone store certainly don't repat it, the want to sell a new Phone, but this don't mean that a tech service isn't capable to do it, there are infinite examples of repaired iPhones (changing a broken scree, battery, plugs and more). The only problem is to obtain replacement pieces from the brand, but this is what the EU will force, that the brand also sell this pieces, which Apple not always do.