If anything, sealing electric toothbrushes is even more paramount than a phone. The normal use case is literally getting it wet through use.
Also, anecdotally, I’ve never had a sonicare toothbrush battery die. They still last weeks after many many years of use.
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.
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."
> Also, anecdotally, I’ve never had a sonicare toothbrush battery die. They still last weeks after many many years of use.
Sure, these exist too, though it doesn't reflect the majority of items which cheap out on all components, including batteries.
It’s the same for me, the whole toothbrush died two times shortly after warranty ended, with the battery having no issues. Not touching sonicare ever again. The cheap honeywell Chinese whatever brand is far more reliable for a small fraction of the price.