If you go all the way back to Adam Smith, you can find an argument against monopolies. He explains that they inevitably abuse power. Their sway is so great that they influence government and actually hamper innovation.
Back in the late 19th and early 20th centuries the U.S. went through a period of "Trust Busting". Workers fought against the bullying labor practices of large corporations. Government legislated against their restrictive trade practices.
Monopolies allow not only a choking off of competition, but also the consolidation of wealth in the hands of a few. This lack of economic equity in a society does not go hand-in hand with political democracy. It leads rather to oligarchy--which is where, I think, a lot of people believe we are headed today. Or maybe we're there already.
Good topic for discussion.