The problem with Marxism is that there's always a way to game a system. The people who are opportunistic CEOs right now would find a way to be opportunistic as an official in a socialist government, except now they wouldn't be producing as much economic surplus. Marxists tend to hand wave this problem by claiming that a strong democracy would keep this in check. However, the world is too complicated, and people have neither the intelligence nor patience to make sure everyone is a good actor.