One trip thru saltwater will destroy the sheet metal and all wiring. Doesn't necessarily mean the inside parts are bad. I imagine the salted roads can do the same up in snow areas. But my comment was more on the idea of scrapping or crushing. If everything got crushed when somebody thought the body was rusted out, then alot of us would not be able to enjoy this hobby. Down here, when they crush stuff, it's loaded on boats to china. If we see it again, it'll be cheap inferior metal products like we see in ebay and discount part houses.
I understand the idea of thinking that poor maintenance on body parts would lead you to believe there was poor maintenance to the over all vehicle, but it's not necessarily so. You've seen commercials showing brand new cars and trucks driving thru the surf on beaches...2 weeks later, those vehicles will have red flakes and ruined electronics.
A few friends and I used to spend spring breaks camping at a place called Turtle Mound, south of New Smyrna Beach. One of the guys left his brand new 1968 442 sitting on the beach when we all jumped in the buggies to ride up to Daytona to party one night. When we got back to camp, the tide had made it in there was a 1968 442 floating up on each incoming wave. The car would rise on each wave and at the top of the rise, the horn would honk and the lights would flash as the salt water had encroached already into the wiring harness and shorted everything out. A really eerie sight. Along with a huge recovery bill from a tow truck that vehicle was ruined. Some of the internals if not accessible to water would have been ok but the rest was trashed.