You can have both.
> Having a power bank solves the problem of discharged phone for me
There are cases where a power bank isn't a good solution. They big and heavy. Not being able to swap in a fully-charged battery on demand means that I can no longer use my phone for certain things that I used to be able to do.
3.7V - 1.23V == 2.47V. 2.47V / 0.2Ω ~= 30W (12.5A).
You will be able the find your phone on the seabed from the bubble emanating from it.
Though the battery protect circuitry (which is inside the battery package) will most likely cut power before the battery is risks damages.
I wonder if that's responsible for the experiences folks had way before waterproof phones were introduced: they would drop their phone into water, it would shut off and refuse to boot for a significant amount of time (enough to send them shopping for a replacement) and then they'd try it a week later only to find that it works fine. Accelerated by putting it in a bag with rice or other desiccant.
You can clearly see here[0] that the gasket on the flap protects only the USB port from water ingress. The battery and sim are protected by the grey gasket on the rear cover.
The opening for the rear speakers in the back cover reinforces the fact that the flap is there to protect the interface of the USB port and its housing. The rear cover does not contact the USB port at all.
[0] https://www.paulstravelpictures.com/Samsung-Galaxy-S5-USB-Ch...
https://www.gottabemobile.com/samsung-galaxy-s5-water-test-d...
> If you remove the battery, the Galaxy S5 will remind you to make sure the back cover is back in place securely. reply
What's stopping figuring out how the hardware can achieve both battery replace-ability and water resistance?
Phones already have holes in them, e.g. speakers and charger port, and yet are able to survive a water immersion event.
Similar thing with the charger/data port. The outside of the port can be completely sealed up with just the electrical connections going through. The port isn't ever opened, there's no mechanical wear of actually going in and out of the sealed area. Glue it all up but leave the electrical connectors exposed and its sealed.
A battery door is a whole 'nother issue. Starting off, its probably going to have considerably more perimeter needing to seal, especially if its like the doors of yore where you popped off a significant part of the back. Then, you'll need this seal to handle a lot of open/close events and be able to handle the dirt and debris which it will be exposed to. Keeping the device's profile thin gets way more complicated with all of these requirements, and the seal will probably still be less reliable than the seals for the charging port and the speakers/mics.
Nothing, since it wasn't all that long ago that there were phones that did this.
I don't think it's easy to design a phone that would at the same time a) have easily replaceable battery, and b) be waterproof until the first battery replacement.