First, we're dealing with analog, so no copy is the same quality as the master. Only a fraction of what was lost every had been digitally copied from the original master. The rest does survive, but it does so as copies of copies of copies of . . . . That's like looking at a beautiful sunset on an old TV through a screen door. It's there, but it ain't the same.
Second, a large portion of music that was stored there had never been released and was never otherwise copied. That is lost forever.
Finally, we've lost all the other history on those tapes.