Antitrust legislation is about keeping the markets competitive, because it also keeps them healthy. In that context, it doesn't matter if the monopoly position was achieved "fairly" or "unfairly", but only what effect on the market it has.
Coincidentally, this is also how legislation was implemented in this country until 1980s. Which is to say - this is what we did back when our economy was both prosperous and stable.
For a good example of just how much teeth anti-trust used to have before it got Borked, read this court opinion in one case from 1962:
https://supreme.justia.com/cases/federal/us/370/294/#tab-opi...