Unfortunately, it's hard to tell what music will still be played 40 years after it came out, without waiting the 40 years.
There was plenty of music in the 70's the has been forgotten. Paperlace, Terry Jacks, Sean Cassidy, and Leif Garrett come to mind off the top of my head.
But at the very least, every act that got to make a record had a team of people, who were supposed music professionals, that believed in them enough to put money into them. Today, anyone can get world wide distribution with a couple hundred dollars and the knowledge that their girlfriends think they are wonderful.
There are great acts out there, it's just hard to find them is the sea of garbage.