This has always been my major concern with the all-you-can eat subscription streaming music model (I wrote about this some time ago). The scale and volume of (preferably paying) users that these services need to achieve to hit some sort of critical mass and start generating compensation for artists, that is even near previously modest levels, is astounding. It also relies on a almost totally unfragmented market where only a handful of major players can compete, or the whole model falls apart.