ok, so... if you're earning money to make music, and then you stop making money, you will probably have to either cut back on the music, or quit it all together (Cause of that whole day job thing).
So, to sustain artists over the long term, subscribers will need to stay in the system. If subscribers and artists stay, then we get a nice big positive feedback loop. Subscribers pay artists, artists make music, subscribers get music, etc...
So the question is raised, how to keep subscribers in the system. Some services use long contracts to accomplish this goal, I would prefer to come up with a service so compelling that no-one would ever want to leave.
Netflix has a good example of positive lock-in with their rating system. Since I've started using netflix I get really good recommendations based on movies I've reviewed in the past. If the system is constantly pointing subscribers towards good music, then perhaps there is enough there to keep them interested and using the system after they got what they originally came in for (presuming they only came into the system to get accesss to one or two artists).