Capitalism always leads to monopolies

No, no it does not. Competition for nearly everything exists, sure there might not be enough but saying "always leads to monopolies" is a lie. Anyway communism usually has monopolies too, just monopolies operated by the government

will always be in conflict with workers.

By workers I presume you mean employees. Yes, greedy people will try to abuse their workers but as the person you're replying to said "unchecked capatilism is also bad", we have minimum wage for a reason, and then there is also competition and if you have a valuable skill they can't afford to exploit you