Natural monopolies are things like electric power grid and water supply and train and roads, they last forever precisely because they are natural and they were never leapfrogged in history. 'Natural" part stands for nessesary inveatment in infrastructure being redundant and causing zero-sum game.

Kodak wasn't much of a monopoly and certainly wasn't a natural one

Read up on how antitrust actually works, your assesment of USPS is equally inaccurate