Built-in batteries are part of the planned waterproof nature of modern phones, so that's a bit of an odd statement in this context.

If you drown your iPhone XS, you just wipe it off. Removable batteries mean openings, which means intrusion opportunities.

None of the iPhones had removable batteries, and none but the most recent ones are waterproof.

They've been reducing ingress points over time. The removal of the headphone jack was at least partially for this reason; so was the non-mechanical home button.

A battery door is a giant potential ingress point for water.

No, those are just more ways to stop people from opening their phones and doing things like replacing batteries rather than buying a new phone. There are water resistant phones with headphone jacks, removable batteries etc. Shoot even a plug for potential infress points would be fine. Sorry, I just hate the new anti-consumer trend of irreplaceable batteries. Feels like we are back to the days where every phone had a diff charger port. Anything to make people spend more money than they have to. /rant.

Yes, and all of those things mean other compromises.