I assume the artists are reasonable rational adults. If they're on the platform, its economically advantageous to them. If they sold their rights at some point in time for money, or pay their tax debts, or whatever, I assume they did so knowing full well what that means. If Spotify is no longer advantageous to them, they would pull their music off and I may search other ways to listen to it.
We have a system to deal with these sorts of things, it's called voluntary exchange and it's the only way things change. If artists pulled their music, Spotify would have to adjust. In the meantime I just don't have the time or energy to track every individual employee grievance