I don't think a civilization built on a shit load of aluminium scrap is going to develop very slowly. Don't forget aluminium is a wonder metal that's only been around for like a hundred years.
All really important metals can be melted at temperatures that can easily be reached with a well built furnace and natural fuel like wood.
Plus, we already know what we did to get to where we are now. We can adapt that knowledge to get back to where we are now. It's going forward we're going to be stumbling in the dark, but we're doing that now anyway.
All really important metals can be melted at temperatures that can easily be reached with a well built furnace and natural fuel like wood.