While I love the 747, prefer to travel in them unless I can get an A380, if QF do the D check, which is far from certain, I think it would mean no more A380 for QF. Extending the 747 is a gamble for QF, that the dispatch reliability can hold (remember the final years of the 747-338), fuel price remains low and revenue demand is sufficient enough to offset the higher operating costs.
I do hope that QF can expand and sustain a larger fleet of A380 and they do not go down the route of 9 across in Economy for their 787. Of course the realist in me says fat chance of that happening.