With your reasoning, for example, the private prison companies would be valued incorrectly high (as do the companies that make the green lights for traffic intersections, the doors to stores, the electric power company, the wireless/phone/cable company, any company with any kind of monopoly, etc). What you're forgetting is to weigh for the difficulty to replace the company with another (that in some cases may already exist and only needs to move into the now vacant market).