I've got a few thoughts on this, I haven't looked into it properly so I might be wrong, but here they are:
I'm not saying the deal for the artists is particularly favourable to the artist, but i think folk forget the illegal download scene a few decades ago; there was already a push against buying music. Who knows how big that would've been without streaming services. a small bit of the pie is better than none of it!
a very high percentage of people using streaming wouldn't have purchased the music. They're getting a small amount of money, but from a different and wider audience than they'd ever have had
If I buy a cd from an artist, only a small amount goes to the artist. That is a one hit for them. If I like an artist and over the next 10+ years, I might listen to thousands of their songs which overall might result in not alot of a difference to the artist.
If artists are more accessible, they're morelikely to pick up fans, and then the whole snowball thing happens.
Excellent points I'd say!