Airline seats are a perishable commodity. "Rescue fare" passengers would fill otherwise empty seats.
Almost by definition, 'rescue fares' are sold at the last minute, and the efficiency of most yield management systems is such that there are no, or only a few, empty seats. These are usually saved for late booking 'must travel' passengers and thus attract higher fares. So being able to give away such seats at low fares rather than let them go empty is not a realistic suggestion.