I personally doubt it. Exceedingly few markets are winner-take-all. Plus, monthly subscribers are "sticky", so they're not going to leave overnight if one competitor gains a slight edge, and everyone else will have a chance to catch up.
Your point about the labels and streaming services as middle men is a good one too. Labels own all the content, so they have a lot of power to influence the market as well.
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.
What we really need are a change to copyright laws that requires a mandatory uniform-fee-only streaming license for all music, tv and movies.
You vote with your dollar by supporting the kinds of market you would like.
I started using Spotify in 2008 because the concept was what I envisioned for the future and iTunes was an awful practice in buying music. I thought I'd still "download" music on the side, but I honestly never felt the need for it. Sure Spotify doesn't have _everyting_, but it has an impressively huge amount of fringe music that I would never have expected and more than enough music for me to listen to until I die. And it grows.