Mylar audio tape of that vintage had longevity problems. I remember reading about an audio archive that thought they had done the right thing by preserving onto high quality tape of the time and then they found their archives disintegrating. I think they got lucky and moved things to new media before losing everything.

This is always the risk. Longevity testing is often done at high temperatures or other artificial means but cannot exactly simulate 30-50 years of storage. If something is important, it's best to use two different media, and check them over the years.