Never flown this route, but I wonder whether this route is really busy. These behemoths are normally used on busy routes (to e.g. LHR, LAX, SIN in the case of QF) or for hub oriented airlines such as EK. But SYD-JNB ? That was a 787 route, an ideal aircraft for such 'niche' connections, like QF 27/28 to SCL. It used to be 744 jumbos before the 787, probably because of ETOPS restrictions as it is a large airportless water body.
But now a daily 380 flight ? Are these full ?