It's weird to me that your line of thinking is actually a thing. It is not difficult to make a water-tight battery compartment for an electric toothbrush, but also make it trivial to open up and replace the battery. Hell, I just did a quick search for "electric toothbrush AAA battery", and these things exist and presumably work fine.
I feel like modern phones and the marketing around them (mostly from Apple) has pushed this nonsense that it's difficult to make water-resistant or water-proof electronics that still have a user-replaceable battery. Unfortunately this marketing seems to be working. Worked on you, at least. Gaskets, o-rings, and pressure seals are old, time-tested technology.
Admittedly it isn't as easy to make a water-resistant smartphone as it is to make water-resistant electric toothbrush. But it's far from impossible.
A toothbrush doesn't have the same space constraints, so they can just have relatively large seals, and they seal a relatively small opening (enough to fit a AA battery through).
It might well be possible to do this in a phone, but this sort of reasoning to come to that conclusion seems faulty. Like saying "Phones should be able to blow buildings apart. After all, tanks do it, so it's clearly possible."
I feel like modern phones and the marketing around them (mostly from Apple) has pushed this nonsense that it's difficult to make water-resistant or water-proof electronics that still have a user-replaceable battery. Unfortunately this marketing seems to be working. Worked on you, at least. Gaskets, o-rings, and pressure seals are old, time-tested technology.
Admittedly it isn't as easy to make a water-resistant smartphone as it is to make water-resistant electric toothbrush. But it's far from impossible.