Marx published Capital in 1867, according to Wikipedia. Actually, I think that Engels completed Volume 2 & 3 after Marx died. It would be a mistake to think that the "last word" on economics was written in the 19th century. A lot of modern (2018) economic developments had not even happened.

There are still important economists who call themselves Marxists. In their analysis and conlcusions, they are sharply distinguished from those who derive their rosy views directly from the other prominent 19th century economist, Adam Smith. Smith thought that capitalist markets would magically solve all problems and that capitalism created the best of all possible worlds. Marx and Marxists knew better.

But neither Marx nor Smith dealt with macro-economics as practiced today. They never heard of credit swaps or derivatives; financialization wasn't even a word. They may have been able to imagine monetary theory or fiscal theory, but they never saw governments apply them purposefully.

To Marx and Smith, money, basically gold, was a commodity. After August, 1971, it wasn't.

Wikipediasays, "On 15 August 1971, the United States unilaterally terminated convertibility of the US dollar to gold, effectively bringing the Bretton Woods system to an end and rendering the dollar a fiat currency.[4] This action, referred to as the Nixon shock, created the situation in which the U.S. dollar became a reserve currency used by many states. At the same time, many fixed currencies (such as the pound sterling) also became free-floating."

In other words, there was no longer any commodity officially backing the idea of "money." The way was open for all kinds of manipulations and, at the same time, for corruption on an unimaginable scale. There are many fine books describing the newer developments, but one can still distinguish the "magic market" economists from those who see that we are in a crazy crisis-ridden system that cannot last.

In 1867, Marx knew that.