The F15EX, apart from being very expensive, would make a good replacement for the F18Fs. (F111 replacement)
Its a bit late for a F-111 replacement.
Also I don't see the point or value in replacing a F-18 Superhornets with F-15EX. I think we have made our choice for 4.5 Gen fighter, and its the Superhornet. The oldest Superhornet is not 10 years old, and the newest one is yet to be delivered. The F-15 while quite agile, is an older, less stealthy platform, and more importantly another completely unique platform to support.
The order window for the F-35 is looking long. Not only that but the existing fleet will need significant upgrades to be competitive and perform missions that the F-18 superhornets currently do.
I am not sure a single airframe "5th gen" fleet is ideal. 5th gen fighters have downsides and limitations, although they are quickly being addressed, and not just limited to 5th gen and we will see the same sort of issues with 6th gen.
We are seeing slow weapons integration, we are seeing low tempo rates, we are seeing the need for hardware upgrades. Limited spares.
4th gen fighters also typically have a large pool of experienced maintainers and simpler production chains. There are airframes sitting in storage. Some roles are still quite suitable for a 4th gen basic EEZ/air space enforcement, bomb truck type missions etc. Superhornets seem to have operating costs simular to a F-16.
If a big global hot war was coming, and the F-35 production line was booked ten years into the future and the F-35 fleet requiring significant hardware upgrades, I can see value in getting more superhornets. Maybe bringing it up to a two full squadrons to help manage the transition. Its integrated with all our weapons, its got all the latest 4.5gen technologies like networking, advanced cockpit, advanced radar, reduced radar signature, longer life and now with blk III, conformal fuel tanks if we wish. The US has a bone yard with some superhornets and there is a fairly robust supply chain for the Superhornet and will be in the USN fleet for a long while yet.
I wonder if eventually it may be worth phasing them out for more capable trainers in the future as a more capable lower tier. KA T50's with F404 or F414 would be quite a capable aircraft. Our big old SH fleet could then provide a lot of resources for such a trainer if it ever needed to operate at a high tempo.
I know the RAAF trainer has been selected, but there was a rumor that the US was going to Lease KAI T-50 because the backlog and slow production. Also there is a defence review going on. And replacing an existing aircraft with a more capable one isn't impossible.