Std Oil was a monopoly, but they also drove the price of oil down 90% and kept it there for over a century. They literally saved the whales. It is very difficult to maintain a free market monopoly without aggressive price discounting.

But even then Std Oil started to break. Before it was over, they were getting their ass kicked by the oil services industry. They wouldn't offer oil services to smaller competitors who were taking over Texas. The share holders revolted, and got the government to break up their monopoly, separating off their oil services industry. Another example of how on their own, monopolies tend to become bloated and unresponsive to the market.