Regarding monopolies, the argument I always hear is this:
If an entity (company or person) that is rich decides to jack up prices or take advantage of people, they have a monopoly, and they might very well become greedy and be abusive...etc (a big corporation with a history of anti-trust issues comes to mind, like Microsoft).
My usual reply is let the market sort it out. That competition will rise. And that monopolies are usually built by government intervention. But it never sounds convincing, to the person im debating as well as to myself.