Sincerely, as Spotify (and the rest of the streaming services) are forced to make more and more user-hostile decisions to keep their revenue growth curve going up and to the right, it might make sense to jump ship early.
While, on the other hand the business could sell their products on their website and therefore get 100% of the money on their website. Many companies sell on their website for this reason.
And then they also need to deal with all the crap that this entails.
If you don't participate in promotion on social media for your art or business, there is absolutely no contact with new customers unless you run a brick and mortar or maybe rent billboards.
In order to monetize streams on your own personal web site you'd likely need to run ads, charge for user accounts, and/or generate paid memberships, which is harder than pulling teeth even as a well known musician.
Maybe not for your exact use-case. But "open" doesn't need large numbers, market saturation or a monopoly: those are traits that come from investor-fueled business-models.
A mastodon account that has great communication with seven other accounts is a 100% success! It doesn't need access to billions of people, it just needs to fulfill a need: talk to several fine people. And it often does this just fine with a very small social graph, even.
A musician doesn't need a million streams, they need enough revenue to be an income. On spotify (or youtube, etc.) that, indeed, means "millions of streams". But it could just as well be five tshirts sold, twenty-eight .zip files @ €9.88 sold, and three vinyl shipped in a month.
The "numbers" that mark "competing with" faang, really don't matter. We don't need to make billions, we just need to make a good income.
If it has to be "better" then it must improve. But improvement doesn't mean "competing with FAANG". It just means being good enough today, and keep improving from there.
My income 35 years ago was vastly different from today. I needed far less, but it also grew over time. But that doesn't mean I must make millions today to consider my life a success. I'm happy today. Isn't that "successful"?
See what’s happening with journa.host as an example.
But to answer your question directly, artists make money the same way they always did - making art for rich people that appeals to their tastes.
Even though that's the case, I think email is much more open than WhatsApp or Signal, and a big part of that is the fact that it's a federated open protocol.
Actually, the EU has passed the DMA which will prevent exactly that for very large (i.e. Facebook/Twitter scale) providers.
I suspect some companies/features may not be available in the EU because of this, but I hardly lament this if it means the garden walls are being torn down.