Another factor about a fuel-burning plane getting lighter with flight time: Many airplanes have a heavier takeoff weight than landing weight. Taking off is fairly gentle on the landing gear and wings as the load is transferred fairly gently. Landing can be an event; you may not be able to stop above your max landing weight, or the force of hitting the ground could overload the gear or the wing spars.

Battery powered planes don’t get lighter with time aloft.