There are examples of enduring competition of all of those bar city streets within a single city, never mind nationally. Even streets aren’t a natural monopoly. If they were you wouldn’t have patchwork metro areas divided up between different municipalities. The benefits of consolidated ownership would be so great that the Bay area would have unified like New York’s boroughs did to form the city of New York. Private towns and streets work fine. For very large examples see Davis in California or Gurgaon in India. The below quoted text is about utilities like water or electricity grids. The US does not have one freight railroad operator even now. If rail was a natural monopoly that would be unavoidable. NYC’s subway system was built by five or six companies. Local monopolies occur in lots of places but they can’t charge profit maximizing prices because of the threat of entry from other firms. The powerlessness of that threat is a necessary part of the definition of natural monopoly. If it doesn’t hold; if the threat of potential entrance of competitors holds down prices you don’t have a natural monopoly.
> The Myth of Natural Monopoly
> In his 1986 book, Direct Utility Competition: The Natural Monopoly Myth, he concludes that in those cities where there is direct competition in the electric utility industries:
> Direct rivalry between two competing firms has existed for very long periods of time — for over 80 years in some cities;
The rival electric utilities compete vigorously through prices and services;
Customers have gained substantial benefits from the competition, compared to cities were there are electric utility monopolies;
Contrary to natural-monopoly theory, costs are actually lower where there are two firms operating;
Contrary to natural-monopoly theory, there is no more excess capacity under competition than under monopoly in the electric utility industry;
The theory of natural monopoly fails on every count: competition exists, price wars are not "serious," there is better consumer service and lower prices with competition, competition persists for very long periods of time, and consumers themselves prefer competition to regulated monopoly; and
Any consumer satisfaction problems caused by dual power lines are considered by consumers to be less significant than the benefits from competition.
It sounds like it's almost true in every case where you are forced to duplicate components/processes/efforts/structures, plus even with diminishing returns economies of scale helps the big companies.
Of course why the big incumbents get complacent/inefficient, why they turn to regulatory capture instead of R&D, is a different question.
> The Myth of Natural Monopoly
> In his 1986 book, Direct Utility Competition: The Natural Monopoly Myth, he concludes that in those cities where there is direct competition in the electric utility industries:
> Direct rivalry between two competing firms has existed for very long periods of time — for over 80 years in some cities; The rival electric utilities compete vigorously through prices and services; Customers have gained substantial benefits from the competition, compared to cities were there are electric utility monopolies; Contrary to natural-monopoly theory, costs are actually lower where there are two firms operating; Contrary to natural-monopoly theory, there is no more excess capacity under competition than under monopoly in the electric utility industry; The theory of natural monopoly fails on every count: competition exists, price wars are not "serious," there is better consumer service and lower prices with competition, competition persists for very long periods of time, and consumers themselves prefer competition to regulated monopoly; and Any consumer satisfaction problems caused by dual power lines are considered by consumers to be less significant than the benefits from competition.
https://mises.org/library/myth-natural-monopoly