Airline operating costs are capital to buy the gear, fuel to run it, crew to operate it and maintenance to preserve it. Note this excludes indirect costs such as management, marketing, insurance, taxes and fees.
Capital is currently dirt cheap, fuel is less than it was, maintenance is getting reduced by a younger fleet with increasing equipment MTBO, so crew is the stand out variable. It may be small on a $ per seat mile basis, but in the aggregate
afaik, it is 15-30% of direct operating costs. That makes crew costs a priority target.