BNEA320,
MLW ? After 4 to 8 hours flying ?
I have no idea what weight QF7 was being operated at that day (or any day for that matter) but here is a little bit of mathematics on the A380 for you.
A380 (figures extracted from Wikipedia)
MTOW: 575,000 kg
MLW: 394,000 kg
MTOW - MLW = 181,000 kg
So based on the above if QF7 had departed at MTOW (not saying it actually did) it would need to burn off and/or dump a total 181,000 kg of fuel prior to landing. You state that it was 4 hours out of SYD so excluding fuel dumping it would have had to be burning fuel at the rate of 45,250 kg/hr to be able to achieve a landing at MLW at NAN. Now I don't fly A380 and I have no idea what the typical fuel flow rate of one is but I dare say it is nowhere close to the above figure.
So excluding other factors already mentioned about engineering, aircraft spares and alternative crews I think your thought process on this is very flawed.
Perhaps think or research (preferably both) before you post, but then again you might be just wanting to stir the pot...