I think the point is that Spotify is so convenient that you essentially own any music on it, especially if you have premium, potentially making it pointless to buy the album for many customers. Prior to Spotify and similar services, if you wanted to listen to music legally (which piracy doesn't give you) and conveniently/on-demand (which the radio doesn't give you), you had to buy the album.

(I'm assuming your point was that Spotify probably pays at least as much per listener as the radio.)