About 300K people fly United every day [1] and for the most part have 0 problems (that other airlines don't have) whatsoever. This one bad incident makes the news (the definition of news) and everyone thinks United is horrible and they will never fly them again. Also remember it was the police who dragged the guy off the plane not United. Once the police got on the plane it was out of United's control. All United did was make a phone call to the them (to boot passengers off for their own employees which was wrong), so I feel people should be directing more of their anger at the police instead of United (although some anger can be directed at United too).
[1] extrapolated out of my ass from 2015 data but should be close https://www.rita.dot.gov/bts/press_releases/bts018_16