Historically, abusive monopolies were created or protected by the state. They exploit licences, concessions, contracts and infrastricture created by the state, and state protection is the only way in which new competition cannot arise. 

Monopolies in a free market are relatively rare, and when they do arise, are generally short-lived. As long as they serve their customers better than anyone else, they deserve their monopoly. As soon as they stop doing so, new, hungry competitors always cut them down to size.

The danger is not so much a market without competition, but one in which no new competition can arise. And only the state can create such conditions.