That entirely depends on how the power balance between content owners and streaming services develops. If labels deliberately protect themselves from a streaming monopoly by fostering competition via good deals for smaller competitors it might become a very balanced market. But if a dominant streaming service manages to press for better deals than smaller competitors, then it could be very much winner-takes-all (with some irrelevant indie for indies sake alternatives sprinkled in between). Labels might actually decide to deliberately let a monopoly happen, to keep the total annual "music tax" per listener high. Spotify is in a business where more hinges on smoky backroom deals than in arms manufacture.
I can’t see the labels being keen to let any one service get a monopoly as it risks them being played off against each other like Apple did to Sony a few years back. I do agree that the music industry is incredibly opaque and how really knows what is going on.
How could a streaming monopolist play content monopolists? There is only one place to call if you want to license a given artist and fans will tell you that you can't substitute one rapper with a different one. The music business is more about identity than about chords and rhythms. "Sorry, no hits for your search term The Beatles, but try this nice, barely used Ballad of Bilbo Baggins, you will like it because it is also old and by a famous person with a funny haircut", that won't work.
Only independent labels without established names, who need the exposure from streaming as much as they need the money would suffer from a streaming monopoly.
