I'm not sure I buy this. I think it's in the airlines' interests to minimize costs (fuel) by flying as efficiently as possible (maybe slower - maximizing air-time). I would be surprised if in-flight spending increased meaningfully over a flight that is ~15% longer.
Yeah, I strongly doubt that in-flight spending on concessions and duty-free catalogs pays for the jet fuel expended over any appreciable time period during the flight.
