I'm not buying that, because that era was also when studios sold their back lots and tons of props -- in short, they were dumping history for a quick buck. Doing better on that score were the record companies (granted, tape reels don't take up as much space as film, but there are lots more reels...
This whole argument is testament to the damage that can be done by those shortsighted studio execs who just throw away footage cut from the films. Not to mention the ones who let completed films just rot away (can you say The Alamo?). The original footage, shot at great expense, is gone forever.