I'm sure I've said this elsewhere but there is nothing really much wrong with older aircraft, with the caveat of good maintenance. In most cases, even though QANTAS has offshored a lot of their major maintenance, everything of importance is overhauled or replaced at service intervals
I've flown a lot on old DC-3s, DC4s, even de-mobbed C-47s (military DC-3 really.) Never had a problem of note. Also biz-jets in the 20 plus year age bracket, similar outcomes.
The Qantas fleet is not really THAT old and with the 787s coming on line the average age will tumble.
The most important issue with fuel consumption is the engines, which are regularly replaced and the engine manufacturers keep improving their product so consumption improves. Sure you can save a bit on lighter weight and improved aerodynamics but not by as much as improved engines.
The oldest aircraft in the fleet would be the 747's incredibly reliable aircraft and the big people mover, the A-380 has not really worked out for them. Nonetheless QANTAS has been progressively retiring the 747s, mostly -400s left now, the most efficient of the marque and still getting improved engines. The 787s will be even more efficient it is true and able to fly very long routes, like Perth/London non-stop and even longer routes if required.
That has a big impact on the cost of flying as well, less landing fees, less need for refueling costs, less need for engineering at stopovers, it all adds up.
Have a look at this article: https://www.ausbt.co...-747-jumbo-jets
You can see they are modernising at quite a rate.
The airline business is complex, I think they are making some good moves.
I'm not especially a fan of QANTAS, especially their CEO but they do have the best safety record in the air,still..