To play devil's advocate, consider how many people you know that work in so-called "bullshit jobs".
TBH, I know plenty of people whose jobs are not that meaningful - yes, some of them create "value" in one way or another, but sometimes even if they create value (indirectly?), maybe it's a bullshit sales job where they are primarily tasked with convincing people to buy their product when an open-source or less-expensive option exists elsewhere. Think "nobody ever got fired for buying IBM". You might choose Expensive Enterprise DB over Free And Open Source DB if you are a purchasing manager and don't want the blowback of choosing something with less history. And the people who sell Expensive Enterprise DB might have a salesforce of thousands of twenty-something's who, for the most part, are not technical experts, but spend most of their days propping up the product and convincing people to buy.
I don't think the theoretical perfect supply/demand curve always matches up with real life. Maybe - in the long-term, it does, but sometimes things fly under the radar, maybe for decades, before the business cycle or something else forces a correction.
I don't disagree with your statement, though. If someone wants to earn a living in music, or at least make money, they need many of the same qualifications any startup needs. 1) They need to understand what they are selling and who is buying (product and demographic). Image/Brand is everything (especially since Instagram). ; 2) They have to be able to market - find a launchpad. Whether this is done in-house or the musician is lucky enough to find someone early in their career that signs them to PR / etc., it has to be done. 3) They've gotta be good at what they are doing. Not everyone has to be a Julliard-trained pianist, but they need to be good enough that people want to listen.
Economically, there are certain genre's that (at least to me) seem to immediately be a problem for actually making a competitive living off it. On the other hand, genre's like pop may be incompatible with someone's age - you won't see many 40 year olds releasing their first album to compete with Bruno Mars, for example.
I don't know, though. Music is less important to some than others. I know people who have literally no preferences and just turn a radio on to hear something, and I know others who spend their days in studios writing and recording and living and breathing music.