Just to clear something up, US law requires anyone seeking a copyright to submit the at least one copy of the material to the Library of Congress (if it requests one). Lucasfilm can't get to them. Same goes for whatever is archived at the BFI.
I also doubt Lucasfilm has actually allowed the negatives to deteriorate. Think about it. This is a guy who actually PLANS on upgrading and repackaging these movies for as long as it remains profitable. Sooner or later, he'll need that stuff.