"If not enough pax step forward, deplane EVERYONE."
You don't think that on a Sunday night that would make the incident that actually took place look like drinks before dinner? Can you imagine the video footage of removing by force a plane-load of passengers, including children, for the convenience of four United employees?
You ask the passengers to deplane. They say, en masse, "**** off". Your move.
Remember, the opening task was to move four staff to a location within driving range: unless the red mist has descended and you've lost sight of your objectives, how does provoking a riot/sit-in to prove your point help? This wasn't about debating the rights and wrongs of the power of airline staff over passengers, this was about providing transport for four people in a reasonable timeframe. Wouldn't a limo, or a NetJet, be less effort than a riot on global TV? The actions of United staff have already made footage of the beating of a passenger a global sensation (it his the 10 o'clock on the BBC, for example, and is now Big In China): imagine what starting a riot on a plane would look like. Why are United staff so keen on "showing who's boss" and less interested in just solving the problem?