As Martin Luther King, Jr. is quoted,
the arc of the moral universe is long, but it bends toward justice.
Capitalism is not democratic, and in its pure free-market, profit-seeking form, it is technically amoral (if a moral CEO passes up a legal, but immoral profit, a slightly less moral CEO will remove that inefficiency from the market).
But you don't need economic theory to prove capitalism is undemocratic. Our own work place experiences are rife with examples. Decisions are generally made by the higher-ups, and frustration among middle managment and the the workers reflects this undemocratic structure. Everyone has their own stories to tell about this.
On a global scale, transnational corporations fly into a developing world town, cut a deal with a the elite, who typically control the government, extract the natural resources, and pocket the wealth while the mass population gains no benefit at best, or is injured or killed at worst. Same undemocratic phenomenon at a large scale.
After the American Revolution, the Federalist Paper #10 was very open about the fact that a small number of wealthy people held great power. Even if pluralistic democracy was desired, the wealthy elite wasn't going to give up their power immediately. The U.S. Constitution was written with this in mind, to protect the power of the "minority," but to also allow a slow evolution and transition in power to the populace. So the theory goes.
However, corporations came into prominence after the US Civil War. They consolidated power, influenced the centers of government, and have corrupted the evolutionary process described above. But, that's only temporary; King's quote still rules, assuming humans don't go extinct first.
There are more classic reasons given for the eventual demise of capitalism, such as two reason predicted by Marx and Engles :
The first reason is that capitalism basically leads to overproduction. The aim of capitalism is to make everything cheaper and faster. This will eventually lead to overproduction, once the means of production how become efficient enough. The markets will fail, the prices fall.
We see this playing out with the great China production phenomenon. We see capital seeking labor markets, raw materials and sales markets around the world, and creating blowback in the process. This reason leads into the second reason:
The management class will not be able to maintain living standards for the working class. The working class won't stand for it, and the system will change.
Efficient production itself leads to layoffs. US corporations moving jobs to China mean these layoffs are permanent. True, we could produce other "things" in addition to the things that are produced so much more efficiently, like environment-saving services and technologies, but these remain outside of the market, and there is good reason to keep environmental protection outside the market: consolidated wealth can afford to pollute or destroy the environment; only a collective, democratic decision can make environmental protection sacred.
Another argues that DEBT is why capitalism will fail. Others argue that lack of morals is why capitalism will fail, but it could be argued that the lack of morals will simply let capitalism reach its natural conclusion, that is, the reasons given by Marx.
With its eventual demise in mind, one has to wonder about the alternatives to capitalism.... a topic for another time.
1. Sourced on Marx.
why will capitalism fail?