I'd love a return to the old snap-fit plastic cases. Besides the ease of battery replacement, those phones seemed much more durable. Maybe because there was something that gave way on impact? I remember watching in horror as the plastic cover of an old LG shot under the display shelving at Home Depot at approximately the same speed the phone hit the concrete floor. I never used a case with one of those phones, nor did I ever crack a screen. It was tricky digging that cover out from under the shelving, however.
I'm not sure dust was ever really a problem for most people. Although, for those in sandy/dusty environments (deserts, some industrial situations, etc) I bet it was a problem.
More to the point, though: I don't think it has to be an either/or choice. Casio makes a crapload of 200M water-resistant watches that sell for $50 or less. This includes both plastic (G-Shock, mostly) and full-metal models (MDV-106/107). The secret is (gasp)... a frigging thin rubber gasket. I frankly don't see any reason why we can't have this level of water resistance in a phone.
We would have to sacrifice thinness and lightness, but not by much. I think a lot of people would happily make that trade.