It's a conundrum for sure. It's a similar flight time JNB-SYD as it is to Paris, London, Frankfurt etc. It's the only place I go where you see more quadjets than twins for those 10-11hr flight times. BA operate a mix of 747/380, Air France operate 380s, Lufthansa operate 748i, EK a 380 (but then again they operate 380s everywhere). You see very few 777/787s apart from African operators and various cargo operators who are presumably enroute to other African destinations. Don't know if it's purely for takeoff performance or perhaps those markets support the higher loads from premium carriers for larger aircraft. You would hope a twin capable of flying for 18 hours, only flying 11, would be able to uplift sufficient fuel for a full pax load and still achieve the required performance. It can be very marginal on the 747 in the summer, particularly if there is adverse weather forecast in Sydney. I'm sure the smartest guys in the room have considered all these factors though.