Look into how much they get at present from regular airwave radio.
The difference being by tuning in to radio you are given the songs as wanted to be played by the station/music companies. They are near using it as advertising for the artist.
When people have access to the songs they want when they want to listen to it, even with adverts, it removes the need to purchase. Especially with the likes of offline playback and remote access over the iphone or android system using their phone connection.
Why bother buying music again?
The small amount of money being produced from this for the majority of content will only last so long. Unless they are shown direct relationships with people buying the music from having heard it on spotify then the big companies wont stay happy with the set up forever.
Either the price will go up, the amount of advertising will go up or the music industry is going to accept a much smaller intake of money than they are used to.
As I said this is a nice system and in the small scale it provides a good way to promote artists and songs. But as accessibility to the service increases the purchase will not be needed.