There is a simple solution. Put aside positive space for 4 crew members on every flight. If they are not used they can be awarded to any non revenue passengers

There is a simple solution. Put aside positive space for 4 crew members on every flight. If they are not used they can be awarded to any non revenue passengers instead. Trust me there are always non revenue passengers for every flight. Plane still goes out full and no paying passengers are asked to get off.