No, it's like saying that when you buy your favorite brand of ice cream at the store the grocery store keeps most of the profit because without the store the ice cream manufacturer wouldn't have a place to sell to the masses.
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.
Saying Justin Bieber gets some of your money isn't really the whole story. Justin Bieber's songs are streamed billions of times a year, so his music accounts for a very significant percentage of total streams, and consequently use of the Spotify platform. Thus, he is bringing an outsized amount of value to Spotify, so he rightly is deserving of more 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.
Are you sure you are understanding the argument being made here?
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)
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.