Music Streaming Impact on Producers
Remember that rivalrous goods mean that someone else consuming it takes some benefit from other people using it--obviously not true for music unless you are a very spiteful consumer. Excludability means you can prevent others from listening to it. Once one person buys it and they start blaring the music, they can't stop others from listening to it, unless they have headphones or something. So it could be considered excludable, but music streaming is often used for parties and such. There is also the issue of torrenting and sharing music online, which makes you the music streamer. So the question to consider is whether music is turning into a public good, and what that means for music streaming.
In any industry, there is always loss from resale transactions. Someone who buys a book from someone else isn't buying it from the publisher or the author. It's just that with music people really like to resale music for zero price (free), which seems odd in a way. Nevertheless, we still see people buy music from musicians because they can be assured of higher quality (so there is product differentiation between a ripped song and the original) and there is probably some weird intrinsic value we get from being a patron of the arts (think Kickstarter or GoFundMe). Is there a negative externality from music streaming services? Probably.
Artists put music on streaming because some see it as better than the traditional way nowadays, just putting your CDs in stores or whatnot. If you get even "less than deserved" from doing that, well...


That's just skimming the surface of this issue probably, so hopefully others will be elaborate better than me.