I've heard some agents claim that airlines wouldn't cancel a flight simply because it was too empty because it leaves the airline with equipment in the wrong city, and the airline's equipment is too carefully scheduled to make ad hoc changes like that. Well, that may be true for some parts of the network, but it isn't true for the cases where the airline runs hourly shuttles between two points. Cancelling a midday CityA-CityB flight leaves the airline with an extra plane at CityA, and short one plane at CityB, but cancelling a corresponding CityB-CityA flight takes care of the imbalance. And in that situation, the airline itself is capable of getting the passengers on the cancelled flight to their destination without using any interline agreements, and without making the passengers >2 hours late to their destination.

That still doesn't mean that the airline would cancel a flight (or a pair of flights) just because the load factor isn't there, but this particular excuse isn't really airtight.