I think you're right, popularity is really the only way to do this. And 0 popularity doesn't seem to mean 0 plays, I think a song with 5 plays 2 years ago would

I think you're right, popularity is really the only way to do this. And 0 popularity doesn't seem to mean 0 plays, I think a song with 5 plays 2 years ago would probably have a 0 rating. This definitely seems like a project where they came up with the idea first and then got as close as they could.