Depending on the reason for the delay the passengers in transit can be a bigger problem for the airline, I think.
I know that the airlines have to pay bumped passengers but there is also cost involved when passengers in transit are delayed.
Some of the costs will probably be paid by someones travelinsurence but won't the insurence company get money from the airline if the airline is responsible for the delay?
I don't try to argue that they will bump other passengers, I have already been told no. But maybe it should be better for the airlines if they did?