Those factories were not producing luxury goods. They produced everyday items for the masses. Things like knives, spoons, tools, textiles, ketchup bottles, shoes, furniture, nails, bicycles, etc.
Standard Oil, for example, improved efficiency and reduced the consumer price of kerosene 70%.
There's no evidence of outsized profits.
They improved efficiency by cutting exclusive deals with railroads and transport companies due to their size which froze out every other company.
SO's market share never exceeded 90% and it declined during the years of the anti-trust trial.
By cutting launch prices 90%, is SpaceX also an abusive monopoly?
SpaceX isn't a monopoly because they have domestic and international competition. Before they showed up, ULA was a deliberately constructed monopoly because Boeing and Lockheed didn't want to compete.
The other reason they're not a monopoly is that they may do 90% of total launches, but most are for themselves to support starlink (another "nonopoly" with no competitors). They have 60% of the US government launch contracts, a small percentage of ESA when Ariane is flaky, amd of course 0% of China and Russia.
Start with "Titan" by Chernow.
SO was basically convicted of "making too much money", and the government failed to convict SO of a monopoly.