Even if Lucas put in writing that it could never be seen again, eventually this would get challenged. You could argue it be legally protected under cultural heritage laws, it's just that these aren't extended to motion pictures under current legislation. It's a matter of time before that happens, whether in ten years or forty years. No one has put the motion before congress because there has been no need, because every other copyright holder has been 110% forthcoming about releasing original versions, pretty much with few exceptions (like maybe theatrical Lawrence of Arabia, but this is just laziness/little demand, rather than deliberate suppression).