If said artist is in such high demand, then he or she can go make their own streaming platform, charge $10/month, and keep 100%. Obviously that isn't the case, so they must get some utility out of using the Spotify platform.
If I buy brand A, the store gets a cut, and brand A gets a cut. Brand B gets nothing.
With Spotify, if I listen to Repartee, Justin Bieber is still getting some of my money.
Again, Repartee is free to pull their songs and sell them directly to you if they think that will bring them more fans and more money. It's kind of funny for two consenting parties who both use a platform to complain that the platform is treating them unfairly.
I think the argument, which is fair, is that it is very weird that if I pay Spotify $10 a month and listen exclusively to a single artist, that artist may get a penny from me and Justin Bieber may get $3 from me, despite me never listening to a single track from him.
Nobody is arguing Spotify can take it's 20% or whatever, people are just arguing that the calculation Spotify uses to distribute the monthly royalties severely penalizes small artists. (because it looks in aggregate across all listeners instead of divvying per user)