Well, "somewhat thin" is subjective so that is no real barrier. Water resistant? I would think you make the phone in two halves, and it splits in the middle. This way both halves can be sturdy enough that a few screws and a silicone seal are enough to keep the water out. Maybe two halves is the wrong idea, but you can still make a stiff back shell that engages in a row of clips along the side that are tapered so as it catches it presses against a silicone seal. Then just a few screws to hold it in place.

Anyway, Apple has solved a million engineering problems. I bet if they cared as much about user serviceable parts as they do about putting LIDAR in their face scanner, we'd see all kinds of clever solutions. Seriously the idea that Apple, one of the most capable engineering and manufacturing companies to ever have existed, could not elegantly solve this problem is a bit far fetched to me. I think the only reason people cannot conceive of it is that we haven't seen it.