DL doesn't generally cancel flights just because they're empty (and the phone agent shouldn't be suggesting this). Doing so messes up the aircraft's schedule and creates a situation where planes aren't where they need to be. OTOH, it can happen that if an aircraft goes mechanical near departure time, the airline takes an aircraft from some other route that's more lightly booked in order to inconvenience fewer people when a flight must be cancelled or very badly delayed. In such cases, the number of people booked on the relevant flights would be only one criterion that would be using in making such decisions.

Sometimes when there's a schedule change of just a minute or so or a change of flight number, the flight appears to be cancelled in the computer for a few minutes. At this time, the system can aggressively rebook customers, just like people (usually starting with elites) can be rebooked by VIPR on the day of travel when a flight appears to be a few minutes late. The software apparently panics fearing that connections will be missed, but the result is highly sub-optimal.