But if that capital accumulation is unchecked it destroys the very market forces such as competition and price setting functions through supply and demand necessary for capitalism to work. So capitalists should want to prevent monopolies and concentration of wealth.

Some goes to the capital class, some goes to workers. The Marxist eschatology is that there are pressures that cause the fraction that goes to capital increases over time and breaks the system.

Look at the good deal that the UAW has gotten for auto workers in the system, both US car makers and the union are pretty happy right to keep this system in place and shrink in the face of technological change like electrification not to mention abandoning small cars for large cars that are profitable for now.

(Funny how I often I see "good old boys" driving Asian compacts because they can afford Asian compacts, and I see office workers driving big-ass trucks)