The problem is not the existence of monopolies, but the fact that the actions of their owners, with regards to what they do with their power, cannot be controlled, and that they will default to wringing every cent they can out of consumers, which is just bad for the greater populace. What if we let companies run wild, and one of them gets a monopoly on, say, water or electrical infrastructure? Everybody will always pay, and an unregulated monopoly will run people into the ground because they know their product is needed.

And yes, their are localized monopolies on such things today, but fortunately government is there to back us up to prevent them from abusing us completely. What would happen if they were allowed to run rampant? Simple fact of nature: predators eat their prey.