I don't want to HEAR about the airlines fuel costs or profit loss

It's my understanding that they are required to make a certain number of flights for routes they are registered/licensed for, or they can lose their ability to fly those routes... excuse me if my terms or wording isn't right, but I think you get the idea.
Years back, my family was the only passengers on a DC10 to Honolulu. Kind of fun- had the full attention of the entire flight crew (lots of freebies). The kids got to play Nerf football against the attendants in the empty aisles. They explained about the routes and the necessity to keep flying them; when I asked why they didn't downsize the plane to something more economical, they said it was because this size plane was needed at the destination for a full return flight. I guess they usually have their reasons, as silly as it may seem sometimes.