(replying from my S5..) The rubber flap is only on the charging port.

Also, I can't figure out how there could be more than one way to replace the battery. It's incredibly simple.

So you don’t think requiring the rubber flap to be closed to have water resistance is a problem?

Samsung even had a warning display on the phone when you turned it on to make sure the battery was secure.

And my galaxy s8 has usb-c and is waterproof, so a robust waterproof connector is a solved problem, they even have moisture detection by measuring resistance between a few pins.