Do not forget one of the main reasons why the SH was purchased. With the USN willing to give up order slots to the RAAF, and with the aircraft similarities to existing RAAF fighters, the FOC date for SH was/is ~2011-2012 IIRC. Different aircraft, like the Typhoon, Gripen, F-15E, F-16, etc etc would take a longer period of time to enter service since there would be more significant differences with the existing RAAF inventory so more training and tools, etc would need to be purchased, and then pilots and mechanics would need a chance to train to operate them effectively. Given the proposed retirement of the F-111 due to service issues the ADF feels it would encounter with the F-111, a retirement date of ~2010 was selected and a followup aircraft would see service in the RAAF.
-Cheers