Marx did not understand the structure of capital, and his understanding of value was also incorrect. The labor theory of value, like the cost theory of value, does not accurately explain reality. The entire notion of exploitation was based around the labor theory of value, and once you recognize that subjective value theory is much closer to the truth, the entire economic model of Marxism collapses.
Prices of things are set by the law of supply and demand, not by the amount of labor that goes into making them, or by the subjective opinion of the purchaser. Buyers try to get the lowest price, and sellers try to get the highest. Labor and subjective value might represent boundaries, but that's it.
"Exploitation" implies immorality. That human labor (physical and mental) produces all goods and services is a simple fact. It follows that people who don't work, whether because they're unemployed or wealthy, too old or too young - or whatever other reason - are subsisting on the efforts of others. It also follows income derived from profits is unearned income, which is what the tax code says it is. This is meant to be positive, not normative. It's not a value judgment. (Though personally I think the development of a class of extremely rich, unproductive people is probably unhealthy.)
What's important is that when too much of the income goes to the owners, rather than than the workers, workers can afford to buy less and less stuff. When they can't afford to buy stuff, effective demand for the stuff they're making goes down. When there's not demand, workers get laid off, which decreases demand even more. It's not a cycle that automatically ends by itself. It's an enormous, unnecessary waste that could be fixed, given the right policies.