For iPhones when sealing them back up we replace the water resistant adhesive around the side to ensure the water resistance is up to spec. iFixit actually does sell replacement adhesive if your looking for a trusted parts source or you can find the replacement adhesive from other parts venders, it is technically different from apple’s adhesive but even an Apple technician would struggle to notice the differences unless they had a lot of experience and knew what to look for(which I don’t), what a technician would look for instead is if the internal water indicator sticker had activated which indicates water damage.

I should also note that all water resistance materials do lose the resistance over time so while a user replaceable battery is possible while having water resistance the worry is both how do you make completely sure that the back(or bottom depending on how you make the release location) is not only completely secure but also that the user knows it’s secure and if not can fix it but also that the user can tell when the water resistant material has worn down(for user replaceable it’d likely be a rubber gasket) as if your often opening and closing the back, say to replace the battery mid day, you’d be applying extra force against the gasket that can wear it down quicker.

For companies there’s a real worry on if the user will blame themselves or the company, Apple for example generally under reports it’s water resistance to prevent customer from being able to come in and say “hey you said my phone was waterproof(*note many customers can get confused between resistant and waterproof) but now it isn’t working”. As an example with the Apple Watch ultra, Apple rates it (and the dive app) for the water pressure down 40M(about 130ft) but it’s shown it can go way past 40M and the dive app will record the data bellow 40M but they’re trying to avoid an unsatisfied customer so by under reporting they’re less likely to have a customer coming in with water problems. Another example, the iPhone 6S was actually water resistant, it had the adhesive and everything however it wasn’t until the iPhone 7 that they advertised it as water resistant because they didn’t want to have any negative customer interactions if the adhesive layer wasn’t good enough and as such they got a whole years of real world data(such as a drop in AppleCare tickets for water damage) before they said anything.