For "piaci monopólium", you could use "competitive monopoly" or "market-driven monopoly".

Here are references for local natural monopoly:

Local natural monopolies are less monopolistic when communities themselves compete with one another. In a county or metropolitan area, folks can then "vote with their feet" and choose communities with efficient utility and transportation services.
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Where competition is not practicable and strong natural monopoly elements are present, services will be provided by a regulated natural monopoly, much as they are under the current monopoly franchise system. Where competition is promoted, policymakers will ensure that market power is sufficiently constrained to allow competition to flourish. Such constraint will include actions to prevent utilities from using their natural monopoly activities to gain marketing advantages