So Spotify (as with most startups) is currently not profitable. In fact, in an interview, Spotify's CEO stated that their focus is not on becoming profitable but user growth. The idea behind it (I'm guessing) is similar to Facebook in that they will become profitable just by the sheer number of users. I am unsure as to how successful they can be in doing this.
On the other end of the spectrum we have iTunes. Now, whatever your opinion might be on their business model or the entire concept of buying music a la carte, the fact of the matter is that it is profitable for Apple.
So while we say subscriptions is the future, the bottom line is there is no monetary incentive. So does Apple change? Or will Spotify break before that?