This is incorrect. There are many industries that lead to natural monopolies in the absence of government breaking them up. Anything with incredibly high fixed entry costs, few substitute opportunities and low variable costs tend toward monopolies (at least regionally).
Telecoms only had a monopoly when the government enforced it (long distance AT&T). Phone companies often had local monopolies because cities would enforce them.
People can (and did) build more roads and bridges if someone was charging monopoly rents.
All tend to see some large realm over which there is an increasing return to scale (if there weren't, monopolies wouldn't form), and quite often a single entity may be established over multiple networks or modalities (e.g, Google controlling search, online advertising, and Web browser development).
I've been hard pressed to come up with examples of monopolies which aren't describable as networks, though that description may not at first be obvious.
Non-government-sanctioned monopolies exist as (illegal) drugs cartels, criminal syndicates, and warlord hierarchies, in which growth and maintenance of the network is typically obtained through direct (and non-legally sanctioned) force of arms, coercion, or intimidation.
Care to provide a list of these industries conducive to natural monopolies that have monopolies broken up by government?
My counterpoint is that without strong independent institutions preventing them, the companies themselves will enforce the monopoly using private security or police forces. If the government is too weak, the company will buy it. If there is no government, the company will form it's own government.
Like company towns- that's the natural end state of an unregulated market. The biggest company in the area can just buy everything and make their own laws.
Under US regulation:
"Monopolization Defined"
The antitrust laws prohibit conduct by a single firm that unreasonably restrains competition by creating or maintaining monopoly power. Most Section 2 claims involve the conduct of a firm with a leading market position, although Section 2 of the Sherman Act also bans attempts to monopolize and conspiracies to monopolize. As a first step, courts ask if the firm has "monopoly power" in any market. This requires in-depth study of the products sold by the leading firm, and any alternative products consumers may turn to if the firm attempted to raise prices. Then courts ask if that leading position was gained or maintained through improper conduct—that is, something other than merely having a better product, superior management or historic accident. Here courts evaluate the anticompetitive effects of the conduct and its procompetitive justifications.
"Market Power"
Courts do not require a literal monopoly before applying rules for single firm conduct; that term is used as shorthand for a firm with significant and durable market power — that is, the long term ability to raise price or exclude competitors. That is how that term is used here: a "monopolist" is a firm with significant and durable market power.
<https://www.ftc.gov/tips-advice/competition-guidance/guide-a...>