I would think at that point they would try and send passengers onward via LH or other carriers. What a disaster. And the passengers on the return flight are hosed too. Probably costs what, $250K for an incident like this?
Should United let someone die or be seriously debilitated because "oh no we would have to cancel the flight"? I understand that you're very likely not of this viewpoint but I just want to point out that that is the alternative.
There is no price for a human life.