I'm pretty sure that Qantas management are quite aware of the relative cost/benefit of new aircraft cp increased maintenance/fuel burn - but current oil price forecasts are pretty flat
The average fleet age is 10 years - higher than it was but these aircraft are good for at least 20 years service so no need to go out and replace the lot . Better to buy steadily - or wait (like BA and Ryanair) for the airline market to soften and buy then. The QF fleet is a lot younger than a lot of big airlines
There isn't much evidence that the SLF care what they fly in - as long as you keep the interiors up to date - otherwise how would the DC-9 (sorree B. 717) stay in service?