In economic terms, the supply of music is vastly increasing – a result of dramatic drops in the costs of creation and distribution combined with many more creators.
Part of the value of music is as shared cultural reference.
I am writing this in a coffee shop. A few minutes ago a conversation broke out about the Spice Girls. Multiple tables joined in to comment on the differences between Ginger, Scary, Sporty and Baby. It was a moment of bonding and community. People who had been staring into their private worlds had a chance to meet.
This was only possible because we all knew who the Spice Girls were. When there is more music, less of it is widely known, so it can’t serve as a landmark anymore. With increased supply, the value goes down.