The Super Hornet was only purchased to fill in for the F-111 that was retired earlier than planned. If you think that the RAAF is only going to be involved in wars that only require strike missions against poorly equipped caliphates then your argument is valid. If the potential exists for peer to peer wars against states equipped with 5th gen fighters then equipping the RAAF with more Hornets is equivalent to using the Wirraway as a frontline fighter in 1941.