This question assumes that Standard Oil didn’t engage in anti-competitive practices after they gained a monopoly position which they absolutely did. On the run / don’t have time to send links but this is common knowledge and easy to look up. It’s worth noting that a monopoly alone (a definition for % market share) does not meet the criteria for antitrust. Antitrust carries an extra burden of the company abusing its monopoly position eg what Standard Oil did and arguably, what Apple is not doing.