Yes, I suppose if you ignore the single largest factor that contributes to Qantas holding aircraft for longer than its international competitors, then it is pretty easy to criticise its strategy.
Sadly, in the real world, the ATO has set a 20-year depreciation period for passenger aircraft, whereas other countries have much shorter periods. Qantas could write down half the value of every A380 it owns by scrapping them after 10 years (since there is almost no secondary market). But, unless each A380 is burning (say) 5-10m a year (depending on your depreciation method, and which year) fuel and other OPEX than an equivalent replacement (assuming a 200m actual purchase price), that'd be financially questionable move. And thats before considering revenue potential.