If you actually look at the history of Standard Oil (including the exposé by Ida Tarbell which arguably depicts the strongest case for the existence of a monopoly), you will notice that Standard Oil's market share was already on the decline by the time that the US Supreme Court ruled that Standard Oil had to be broken up. The break up wasn't even very effective in Standard Oil's case; the modern ExxonMobil is the result of mergers of some of the largest of the daughter companies.