I submit that the long term issues we are having in the united states, are in fact the result of short term solutions. Also, standard oil controlled more like 90% of the market, so in that context they had lost more like 25% by the time the trial concluded.
I'm pretty pro-market, but Standard Oil used a lot of techniques to fix prices. Having a lot of refinery control and the ability to undercut anyone who didn't play ball with them (and thus kill the small independent) did happen. They didn't need a true monopoly, just enough power to tank any smaller company that tried to undercut them.
If you're interested in the subject, The Prize: The Epic Quest for Oil, Money, and Power by Daniel Yergin is a fascinating read.