You're correct in thinking that a publisher who gives an artist a big advance and ends up making a decent return form the finished work may not share a dime of

You're correct in thinking that a publisher who gives an artist a big advance and ends up making a decent return form the finished work may not share a dime of that with the artist. But given this success, the publisher is also a lot more likely to underwrite new work, which is very much to the benefit of the artist. Meanwhile, the artist is in a position to increase their demands related to this follow-up effort, so saying that the publisher's success has no bearing on the artist's isn't correct. Yes, the artist has to negotiate well and produce more to realize that benefit, but the absence of a clear path isn't the same as the absence of any path.