I think the most convincing Marxist idea related to exploitation is his conception of “wage slavery.”

In bondage slavery the slaves are in economic terms not labor, but capital — like machines, or draft animals. If a person is employed for wages or a salary, that person is being used as a machine — or a draft animal.

Even in bondage slavery it was not unknown for slaves to become rich and even politically powerful. That there are rich/powerful wage slaves does not change the nature of the system.

The inherent nature of a system in which wage slavery is the basis for the distribution of material well-being is for fewer and fewer people to get more and more in compensation and for more and more people to get less and less. The presence of big, strong unions temporarily altered that process. Their demise has coincided with the resumption of its normal course.