I'm not tremendously impressed with Marx's analysis.
Here is my initial premise. Across the entire economy, current consumption is paid for by current production, with any difference buffered by available filled stockpiles and empty stockpile capacity.
If you eat an ear of corn, someone has to grow an ear of corn, and transport it to you. If that person grows more corn than other people want to eat, it has to be stored somewhere until those people get hungry again. And the transportation and storage themselves consume resources.
This is the economic gradient. Goods and services are brought to market by producers, and taken away by consumers. In order to make this worthwhile--remember that bringing something to market is not a zero-resource operation--the consumers must give the producers something of greater value to them than whatever it is they produced.
Because this is sometimes tricky, we all agree for convenience that everyone always wants "money"--whatever it may be--more than whatever it is that they produce.
This works just fine when all producers are also consumers.
Now introduce industrial capital. Now a producer can, via the multiplying effect of capital, potentially single-handedly satisfy the entire worldwide demand for a good or service, at lower cost than any other producer who lacks the required capital. Again, this still works fine, so long as that person can manage to somehow consume all those resources.
But then we run into a problem. What if that person cannot manage to spend that much? The excess has to be stockpiled, according to the premise. Without the convenient fiction of money, that super-producer would have to start filling warehouses with goods and services. (We will pretend for the moment that services can somehow be stockpiled.) But even then, until the imbalance is rectified, that guy will simply keep filling warehouses that can never be emptied. Those resources are lost to everyone else. The super-producer simply has everything he could possibly need or want, and there's nothing anyone could possibly give him in trade without first tricking him into thinking it is better than anything he already has. With money, the problem of warehouse space is solved by reducing all that surplus to a number, which is stored in a computer.
But then we run into another problem. The value of the money is determined via continual market recalculations by the amount currently in circulation. Warehoused money might as well not exist. The super-producer has to keep moving it around, or prices will adjust to accommodate the fact that it is not being spent. He therefore gets less in exchange for his production. Then, whenever he goes to buy something, prices rise again temporarily. So the super-producer "invests" his money stockpile, which is a fancy way of saying that he moves it around from being owned by him in his own name to owned by him in a fictional business name. Or it approximates virtual warehouse space for his owned physical goods and for his owned services. The point is that investment is not consumption.
Whenever someone produces at a level that far exceeds their level of consumption, someone else's production is essentially being thrown into an economic black hole.
That's the problem with Capitalism. It can concentrate wealth so much that the owner cannot possibly consume it all, over his entire lifetime, or even in the lifetimes of his heirs. And until someone in the line of inheritance can become creative enough to embark upon some consumption megaproject, the concentration of capital sucks the life out of the productive economy.
Marxism solves this problem by diluting the concentration right at the source, by distributing the capital production in the hands of more spenders. But then there are no megaprojects without re-concentrating that wealth via government.
The capitalist solution to this problem is to give rich people more expensive things to consume. If, for some reason, they don't want to consume, the system fails. And nowadays, rich people are mostly investing rather than spending. If they do not change their ways, and quickly, the traditional release valve is to take back ownership of resources by force.