Waterproofing isn't a yes/no feature. Modern phones are way more waterproof and dust resistant than the S5. The plastic back on the S5 would quickly develop cracks that would let water in.

Sure, it wasn't perfect, but it showed that it can be done, and it's not particularly difficult either. An improved design could have a metal back panel instead of plastic, it could use screws instead of plastic snaps, etc. (And with most people using cases, who cares about visible screws?)

It was really not waterproof. It was IP67, which is far from the IP68 almost all phones have today. https://phandroid.com/2014/04/21/galaxy-s5-ip67-meaning/

If it's waterproof and you get the water on the inside, it should in fact hold water