"I assume the artists are reasonable rational adults. If they're on the platform, its economically advantageous to them." - Your assumption is wrong though. Artists aren't on spotify because it's economically advantageous for them, they are on spotify because they have no other alternative. It doesn't matter if it's streaming platform A or B or C, they are ubiquitous and as an artist you want your music to be heard, so you have to play the field. In this case, the field sucks and mirrors capitalism, skewing the gains towards the top of the pyramid.