People will just cough up for a paid version. This, ladies is the bait and switch and is what the industry has been hoping for which is migration of paid subscription services. They have never liked ad revenue from free handouts because that is managed by a model not unlike radio licensing. Actual pay per play is a wholly different financial structure.
Remember the big three labels consolidated are majority shareholders of Spotify and they have not liked this free business for a very long time. Apple is the excuse they needed, and Apple knows this and is playing the obvious gambit: they want to first level the playing field for Apple before owning it. Apple just killed a untenable business model. Why should Wonga or Liposuction adverts pay me the revenues for my music?
Despite being easier than piracy Spotify has had a negligible effect on piracy, almost none in fact, but what it has done is insinuated itself into culture such that the idea of mom and pops, not just tech savvy geeks paying £20 a month for the music they listen to seems like a pretty good option and hey, no ads! Now cough up!