People don't care if a song is 50 years old or 25 years old. They like it or they don't. Someone who was 20 in 1990 is now 48. That puts them on the edge of obsolescence to Ad Agencies...
It all comes down to who pays for it. it's not about music or music fans, it's about paying for the service. If the people pay subscription for the music, it doesn't matter how old it is. If advertisers pay for it, then they get to determine who the audience is. I really believe a time will come when all music will be subscription-driven, because the audience is being carved up into smaller and smaller bits. Advertising is about reaching mass audiences, and music is becoming less about the mass, and more about the individual. There are fewer formats that attract the kinds of numbers that advertisers want. Will people pay to hear their music on the radio? We'll find out.