it would actually be more like napster, which nobody remembers anymore.
you'd sign into the site and look for music, all of which would be independent. this music would be hosted on the computers of random people, like torrents are. you could then stream the music over the distributed network the same way you use spotify.
if you're an indie artist, you don't get paid by spotify, anyways. it's a waste of time. don't bother.
rather, the benefit of siphoning out a network for indie artists would be to get people out of the system and try to rebuild an underground. think of it more like tape trading...
somebody would have to police this, of course; you wouldn't want copyrighted music in the service, and people that want to share copyrighted music should just use torrents. but, i actually think this would be less difficult than you'd expect.
i wouldn't sign up for spotify, but i might sign up for something like this.
as it is, what we have is bandcamp, and the question as to whether it's sufficient or not is somewhat open.