They can play their "music discovery arbitrage" game till it grows big enough that their partners have enough incentive to step in and take over their business.
No different than what happened to Facebook app makers. Even Zynga's profit outlook is questionable as it is crystal clear from their latest filing...
Spotify's best bet is to sell themselves at the peak. But they are pretty expensive already.
A more pragmatic music label will charge as much in license fees as they possibly can without killing Spotify.
I think they're losing money on how many songs people listen to vs subscription cost. The advertisement isn't offsetting it.