Again, I think the Super Hornet makes more sense since the RAAF was already operating the Hornet, but the original Hornet purchase seemed to be more conservatism than anything else. In a world without the F-111C, the multi-role nature of the Hornet offers a lot of flexibility, but that wasn't where the RAAF was in 1981.
I also understand that we have the benefit of hindsight. Back when the Hornet was selected, Australia's neighbors didn't operate anything much more intimidating than a MiG-21 or F-5 and it didn't look like that was going to change anytime soon. In the subsequent decades, though, everyone and their cousin was stocking up on Flankers and legacy Hornets don't look quite as capable.
When you're operating the very capable, fast, and long-legged F-111C, though, the legacy Hornet seems like a slight mismatch with too much overlap in some areas and not enough speed or range in others. Had the RAAF purchased the F-15C in 1981 instead (even a smaller number), I think we'd be talking about how much longer they intend to keep flying them, instead of the fleet being retired in 2021. Japan's F-15J fleet offers an interesting comparison that illustrates the continued relevance of the platform, one which would still operate well alongside the F-35 for some years to come.
I understand, no war broke out that needed the Hornet to go up against Flankers in its 37 years of RAAF service, whereas its air-to-ground capabilities were employed throughout the Middle East during the last half of its lifetime, so one could argue that it was the perfect choice for the actual geopolitical environment Australia found itself in compared with the hypothetical regional matchup. Still, I think trusting that you won't need higher-end capabilities over the next 40 years might be a winning strategy if you're in New Zealand's position, but it seems a bit more of a gamble as Australia.
I'll admit, I have a close family member that has personally built and serviced both the F-15 and F/A-18 types over a 40 year career and has expressed a serious preference for the F-15 when it comes to sustainment and serviceability, so I am coming at the topic with that in mind.
Cheers,
Logan