You're assuming that the artist knocks out a few tunes over the weekend, and have no other costs.
Whether he's assuming that or not is completely irrelevant.
The correct assumption to make is that music artists are making music because they like to make music, not primarily to make money. Art is something that almost always suffers when money is the primary motivating factor. Not to mention the fact that music really is a pretty basic and easy art, (well, at least the stuff that's currently popular) and there's guaranteed to be a never-ending new supply for the foreseeable future.
We all have things that we spend a lot of time on because we like to do them, and I'm sure we'd all like to keep making *any* amount of money for posterity for the time invested. But that's not how reality works.
So GP is right. They're making as much as $5,000 a year for the simple act of uploading their music to Spotify. They should be pretty happy with that.
Mostly music takes a lot more effort, time and money to produce than that -- the stuff you want to listen to at least.
It takes exactly as much time, effort, and money as you want to put into it. And putting more of any of those things into it doesn't necessarily make it better. Most of the expensive contemporary pop music sounds like shit, and is easily surpassed by thousands of tracks indie artists give away for free.