I think that with the success of services such as Pandora, Spotify, and the like, it is reasonable to say that the next big thing on the internet is to be able to stream whatever music you want on an on-demand basis. The question is, can anyone afford to do such a thing?
I think that since the world of tech moves so very quickly, if on demand streaming of music over the internet were in some way profitable, we would have already seen someone come up with a plan and put it in to place. It speaks volumes that no one has done so yet.
We've been able to stream music on-demand for at least half a decade by now. While some companies may be slow to adapting and offering services comparable to Pandora or Spotify, it doesn't mean the Internet as a whole is behind the times. There are also plenty of great startups like SoundCloud allowing streaming on-demand, so it's hard to imagine that we're going to be seeing anything incredibly new over the next little while from the on-demand streaming scene.
On demand music will be popular for a while, but it will not be the next big thing. I expect it to faze out rather quickly to be honest. This is due to people having to pay for the service. Most people these days download music for free and already he tons of music on their phones.