Yeah gaskets have existed for a hundred and fifty years or more so it won’t take great feats of engineering to solve this issue. I personally don’t care if my phone is waterproof or not since it’s ingrained in me not to get them wet from years of using electronics prior to waterproofing being common. Maybe adding a removable back will mean the end of camera bumps too.
The waterproofing has been solved, but the real issue is that device size and/or battery capacity is going to be affected. And with the resiliency of modern batteries, you’re making design sacrifices for something that won’t affect most users.
Sure, it was nice being able to swap a fresh battery in on the fly. But these days you can also just get a decent power bank for less money than a proprietary battery pack.
A middle ground would end up being better for everybody. Keep the batteries as they are now, but make the phones a bit easier to open (and use fasteners instead of adhesives).