The usual counterargument is that the Conspirators pay off airlines to compensate for fuel and pay off pilots to keep quiet about it. But that assumes there's no righthearted pilots and executives in the world who would prefer honesty to the money, and it has no bearing on the flight duration.
Right... But since you cannot speed up flights, that would mean that the "shorter flights" (from a flat earth perspective) are slown down (as in the routes are made longer). That would be the case for flights on the northern hemisphere of the earth. So a flight from Sidney to Santiago would be a straight line, while a flight from Paris to Moscow would be made twice as long/slow... That is rather noticable when you are inside th plane. So it does not only imply a conspiracy for the pilots, but also for the passengers who look outside.