I’m not an airline economist , and I doubt if there are any on this site , but I do know that Alan Joyce CEO Qantas has stated many times that the DFW- SYD non stop service could only be operated profitably by a A380 . The DFW and LHR services were Qantas’s biggest earners , both operated by the A380 . They do need to be flown at almost max capacity , although that wasn’t always the case with DFW -SYD , but filling them was never an issue on those routes . Slots at LHR are rare and ultra expensive , so large capacity aircraft really come into there own at these airports , basically uplifting double the capacity of large twin aircraft .