It's not just a question of prices rising. Failure of prices to drop to the level they would otherwise in a competitive environment is also damaging. AT&T is o

It's not just a question of prices rising. Failure of prices to drop to the level they would otherwise in a competitive environment is also damaging. AT&T is one example of such a situation. After the breakup of AT&T, long-distances prices dropped dramatically, and quality improved just as dramatically. And I'm not sure how far your definition of "textbook monopoly" stretches, but the concept of "antitrust" covers not just monopolies, but cartels and cartel-like activity (like secret price-fixing agreements). OPEC is an example of cartel that has been successful in keeping prices artificially high for probably a majority of its existence.