I was wondering about that. I lost my iPhone 13 mini the other day, did the find my phone beep thing and got a distant beep from my washing machine which was on wash cycle.
Surprisingly the phone was fine and works fine after a brief rinse under the tap. It must be hard to combine that sort of water resistance with easy user changing.
Putting the battery outside the water insulation zone might work for that, it's a sealed pouch anyway.
Conformal coating is a little more expensive than gasketing, but it works much, much better under pressure. Motorola does this.
Third parties offer new iphone batteries so it's clearly replaceable commercially
Don't fall for the 'glue cuz of protection' myth - there are and had been water-resistant phones way before Apple started glueing to avoid customers doing their own repairs and them losing out on new sales.