Regarding "better water resistance", there have been a ton of phones released in the past that have had water resistance standards of at least IP 67 and that had a user replaceable battery. That part of your argument doesn't hold water.
My iPhone's battery health has been stuck at 83% for nearly 15 months. It was only 9 months old (purchased brand new) when it reached 83%. Apple will not replace with Applecare unless it is below 80%. Something doesn't add up there.
>better water resistance/proofing/reduced engineering cost
This is a myth. Look at the Samsung Galaxy S5 (IP67) & Sony XP10 (IP68). (Yeah, I'm a big Louis Rossmann fan; I learned this from his videos.)
Even if it were true, are you telling me Apple couldn't engineer a solution to the problem and turn it into a key marketing element boasting their innovation?
Waterproofing in terms of actually being able to use the phone under water or not breaking when it falls into water (i.e. it will switch itself off when submerged and after it dried it will work again)?
There is a technology called “rubber o-rings” that has allowed for waterproof/resistant user replaceable battery designs in cell phones since they came out.
Had to replace an iPhone battery exactly once over the last 3-4 years and it was done for free with applecare, in and out in 30 mins.