New Truck Frames (are they doing it wrong?)

[K10 STEPSIDE]

This is something that has bothered me for a while and maybe I just don't have enough experience with them but it seams to me that the new frame being fully boxed in will lead to rust and they won't last long. Am I missing something or did the designers overlook what seems like a great place for dirt to build up and compromise structure. If an old truck is of the ground it normally has a good frame being channel, but will it become common for trucks to fold like Toyota's?
I started to look in to the old Toyota's that have frame issues and they seem to have a full frame for at least part of the way.