Your comment is interesting, because it points out the fact that quality does vary over time, for all types of goods and services. I think this is one of the strongest arguments for creating an environment where healthy competition exists.

The reason being that if a business/organization declines in quality, competition always exists to step in and serve those whom the organization previously served. When you have government controlled monopolies, private competition is literally killed off because the private competition's tax dollars typically go to subsidize the government entity serving the same customers. This keeps prices low, but it does nothing to address the quality of the services being provided, which oftentimes degrades over time (this is partially why most Fortune 500 companies weren't on the list 20 years ago).

When the government steps in and serves a group of people in the marketplace, there is rarely a mechanism to give consumers an alternative should the government entity fail to provide adequate service.