Supply and demand is probably one of the most basic, crudely simple, and well known economic ideas. It's not a universal, and one reason it is so popular is because of the aforementioned crude simplicity, but it serves a point and has a role in common discussion.
A sort of argument about supply and demand is constructed in defense of capitalism when referring to products, or creations, that fail to sell. That is, it is said that, if someone has a passion to make something, and they do make it, but nobody buys it, well, at the end of the day, the lack of demand is what it is, and the people have spoken, and the creator needs to look to other means. This is seen as a necessary and morally informative facet of the free market system. That it is a democracy, and that if nobody buys your product, creation, idea, or service, then it is you, the creator, who is at fault. Something that meets a demand needs to be created. Not just the whim of a creator with little demand.
On its face, it seems logical. At the end of the day, we all labor 40+ hours a week towards the capacity to make money and in turn convert that money into things we enjoy. And so if something is created that is outside that causative chain, then it is unfair to expect us to buy or consume said thing.
I would argue, however, that this idea of having to create something that meets a demand is inherently immoral given the current construct of modern society. We talk about say, a book, or video game, a movie, or new invention failing to sell because people simply don't like the product, but the reality is that it is not so much people individually choosing to pass on a product, but rather a sort of rejection formatted through years of marketing, consumerism, and corporate scheming. The rejection of, say, a passionately written 100,000 word novel is not made by a wholly democratic, open, and individualistic "invisible hand" but rather by a mob of people whose very wants, desires, and likes, has been formed by forces completely outside of their control. Nobody is immune to it. All of our fashion choices, media choices, food choices, product choices, are fashioned in a large part by the advertising we see, the friends that are thrust into our lives (whose choices are also influenced by advertising), the culture we are born into, the religion and class we are born into, and much, much more. Things that are working at a much larger scale than an individual, micro-level. Thus, the entire idea of expecting someone to do something that meets the current demands of the market is immoral, because it puts an individual or small group in a position of having to cater to entities that are much larger and more powerful than they are, and also pits a creation against the collective identity of a society that is often left to the whims of modern commercialization. Until the system is more democratic and more formed by individual opinions unbiased by these institutions, the idea of having to create something that there is a demand for will always be unfair for the creator.
It behooves me, then, to simply summarize that our nominal decision making regarding the quality of a product is not as open or democratic as many like to suggest, but is instead a function of society and commercial entities at large, which is hardly fair to a small individual who is chasing their passion. That is not to say that products and creations aren't rejected because they fail to meet the interest of people, indeed, that is often the case, and I can not sit here and simply say that everything that has failed or been rejected was done unfairly so. But it is interesting to reflect on the fact that, if you are setting out, as an individual, attempting to make something that you are passionate about, or do something you love, you are not attempting to meet the demand of people, but of the corporations, cultures, and predispositions that have formed in and amidst our society and continue to be pummeled against us. It seem rather cruel, then, to expect anyone to come out on top. Only a lucky few do. Perhaps when someone fails to achieve success with their creation, we can offer them congratulations for trying. Many people never even get that far.