Here are my thoughts...
Aluminum has been used for years for auto and truck bodies. I don't know if Ford used aluminum in all Crown Vics for the trunk lid, but I know the police cars do, so Ford has used the metal in production cars for some time. My Mom's Mercedes has an aluminum hood and trunk. They sound a little different when closing than the steel ones on my car that is a few years older. As to repairs, who does any real body work anymore? They just bolt on a replacement fender or door. That said, it can be worked if one has the skills. A friend who worked in the local Chrysler dealer's body shop 40 years ago knew how to do the work. It was part of his training then.
Aluminum has a protective oxide layer that prevents further oxidation. Salt will effect it, as you would see on window frames of homes near the ocean. It doesn't have the habit of rotting through like steel does. I would think a properly finished body panel of aluminum would last longer than one of steel in the rust belt.
For the inside of a truck bed, plastic would not be good. The heavy impacts of things being dropped or thrown would crack it. For the outside body panels, it seems great. We had a Saturn with the plastic body and it held up great. Rust is never a problem. Plastic has a limited lifetime due to oxidation and internal breakdown (from what a chemical engineer told me), but the life is longer than the mechanicals of the car.
Just a few thoughts from someone who drives 60 year old all steel cars.