A few comments that do not contradict the OP:

IANAL, however the term in law which would be most useful is "market power." Monopoly (or oligopoly) is just one species of "market power." It is the abuse or extension of market power with is illegal. Charging ANY fee for installing an app on an OS might be seen as the extension of market power.

Public utilities are subject to regulation, even if they are not in a monopoly position, (such as two private roads necessary to get to a town) if their market power - separately or together) appears to have led to abuses or inefficiencies (such as unnecessarily large fees.)

The root of "antitrust" law is English common law against "restraint of trade." Statute law, such as the Sherman act in the US, has generally been enacted in order to limit (not extend) the penalties already possible under common law. Common law is an English thing, going back to Henry II, I believe. Most nations have legal systems that descend from Roman law (or similar systems) and don't include common law. (I'm going by a book by Churchill, here, who also was not a lawyer, however.)