Why would the aluminum stock not be useful after 10 years, oxidation?
It might be in specific forms ready to be processed - eg. blocks of specific dimensions or weights. After 10 years outside, it's probably covered in dust and grime, and gotten an oxide layer. It probably isn't immediately sellable to a car factory... But it could be reprocessed very inexpensively and become sellable. The loss of value would probably be well under 1%. People saying "it's so old, it's basically scrap"…
Aluminium has an oxide layer the second it comes into contact with air.