Every pop radio station played them. But there were very few pop stations. You had no choice. You either listened to Classical, EZ, "race", Country or Top40/Pop. The kids all listened to Top40/pop.
But the music industry fragmented after the Beatles. Along with the Beatles were other superstar groups like Zep and Pink Floyd. But after that, radio became fragmented. Now we have urban stations, alternative stations, soft rock stations, dance music stations, rap stations, and at least a dozen other formats competing for the same age group.
Metallica is not an icon of a generation - every child of the generation does not know any of their songs by heart.
Unless the music industry reverses itself, I don't think there will ever be anything like The Beatles or Elvis or Sinatra again.
The Beatles also knew when to quit. Metallica, not so much.