aluminum corrodes also, its not totaly safe from the dangers of corrosion, after a while salt will eat it away, you say its been around since early 70s, well we have trailers around here that are even older than that, all steel and they are fine also, structural steel lasts alot better than sheetmetal on cars.
and the nice thing about steel is that when ya wanna add tie downs or whatever or need to repair it you or anyone with a stick welder can weld it, aluminum requires expensive tig welding equipment to repair properly