Doc Holliday
En-Route
If your theory is that the airlines evaluate the load on each flight and then decide whether or not to operate it, you are mistaken. They do evaluate routes, but those decisions are not made on the spur of the moment, and route changes would cause the flight to be unavailable for sale. No one would show up at the airport and hear bogus cancellation stories when a poor performing route is ended.
Also, they are carrying freight. Some estimates say 10-25% of the revenue is from freight. Even a light load (passenger-wise) can cover its operating cost and not disrupt schedules. Disrupting schedules adds more cost than operating the flight at zero or negative profit.
Here’s something else to consider. Airline aircraft are maintained on a progressive maintenance system. Maintenance planning goes into deciding when a certain airframe will arrive at a maintenance station, and how much time it needs during a layover to accomplish an inspection. Throw a monkey wrench into this, such as a weather delay or cancellation, and it dominoes through the fleet. The airlines want those airplanes to arrive and depart on schedule, or it becomes a real nightmare.