This is actually a basic tenet of economics. The origin of profit is the unpaid labor of workers. How else could someone accumulate such incredible wealth? No one produces a million dollars worth of goods a year, but many receive that much. Economics is a zero sum game. This claim is backed by the laws of physics of our material world (conservation of energy).
Want proof? Work as a coal miner, and see how much coal you can buy when you're done. Even if you do it alone with a shovel, you won't be able to buy what you dug up. Even after factoring in the cost of administration of the mine, etc...
Marx actually was the founder of much of our modern understanding of capitalism. He was more of an economist than anything else, though he basically founded modern sociology. One example of how little credit he has been given is the business cycle. He insisted it was inevitable - there would be overproduction, which would lead to inability to sell products, which would lead to workers being laid off - a feedback loop - people becoming poor because there is too much wealth stored in locked, guarded warehouses. (Oh, the irony, and how reminiscent of today, as people go homeless because there are too many homes.) He was ridiculed for his theory of the business cycle for 100 years. Over and over, the capitalists believed they finally had the solution, and that Marx's theory would finally be proven wrong. Only after the failure of Keynesianism in the 70s was it grudgingly accepted as fact. But no one gave him credit for it.
He covered far more ground than this, of course. And all of his work has been vindicated by history. Reading through what he wrote was surreal simply because of how accurate his predictions were. After reading science fiction novels in my childhood, I grew accustomed to laughing at books from the past, but with his, I did not laugh. The only thing he failed to consider was the full power of the welfare state to suppress the class struggle for entire generations. By softening the harshest realities of capitalism through progressive taxation, welfare programs, labor legislation, and regulation of business practices (most significantly, the food industry), enough of the population was able to live comfortably enough to prevent any serious threat to capitalism from materializing.
Ultimately, Marx's fundamental criticism of capitalism was not about the inevitable stark inequality of its economics (paid according to power, not productivity) and undemocratic nature of its politics (who actually thinks they have a say unless they have money?). He focused instead on its failure to utilize manufacturing capacity (to "develop the productive forces").
The dive in the standard of living during crises has much more to do with the closing of farms and factories than rising inequality. From around, I believe, 1950-1970, the wealth enjoyed by the average person doubled, but the change in inequality was small. It was full employment, and the development of more efficient manufacturing techniques, that led to material abundance.
With capitalism, when you have too many homes, people go homeless, when you have too much food, people starve. Factories workers built by worker's own hands sit guarded by cops so that they cannot get back in to restart production for their own needs. Warehouses full of goods are locked up and mothballed, so that workers that made them, and need them, cannot have them. Fields grow thick with weeds, and those who would till this earth instead starve landless, living lives of poverty. In crisis, capitalism stands fully in the way of production, loaded gun in hand, menacing any starving person who would dare plant some crops. This happened - literally - during the Great Depression, when farmers were kicked off their land by the banks during the Dust Bowl.
The story does not only end with economic disaster. War is also inevitable during crises, as trade wars result by sheer necessity, leaving one country backed against the wall with no choice but to respond with force, as the loser's industries find themselves unable to sell their goods, or purchase necessary raw materials. When they lose the trade war, they turn to the shooting war. Stage one is happening now and stage two is on the way. Europe is rearming.
160 million people died from wars in the 20th century. Far more were injured, and far more still were forever mentally scarred, or were forced to live lives of abject poverty due to the bombing of factories, torching of crops, and slaughter of fellow workers. It's going to happen again if we don't stop it. The path from crisis, to trade war, to war, has been traveled many times.
Those who slander Marx know little about what he actually said. I was once among them (libertarian), wrongly believing that mass murderers like Stalin or Mao were the Marxists they claimed to be. Marx was the most sensible of economists and reading his works made the events of the last 100 years actually understandable - a first, for me. He didn't write about how everything had to be collective property, or how the rich had to be shot in death camps. He didn't sit down and design "perfect societies" on paper - it was actually his frustration with these unrealistic idealists that got him writing in the first place. He envisioned a democratically run society, where men and women, blacks and whites, gays, drug users - everyone - had an equal say in both the economic and political spheres of their lives. He showed in his works how capitalism maximizes the worst in us, and makes good men do awful things (Warren Buffet got rich killing wages, and Steve Jobs got rich by exploiting
horrible working conditions in China). He recognized, rightly, that capitalism is fundamentally flawed and that even if it provided perfect democracy and full equality, it would still use resources poorly and still suffer from crises, and that for us to minimize the worst in us, and make efficient use of the resources available to us, we would need real democracy, and rational management of these resources.