With respect to Australia aircraft procurement, one needs to keep in mind what Australia was/is looking for in specific aircraft projects, when this projects are running, what is available or on offer at these times and/or when Australia needs to have a capability in service by.
In a number of instances, US-sourced kit was the only real contender available.
At the same time, I do believe that a couple of Australian aviation selections were made based upon political (rather than Defence) considerations, and the two which stand out most IMO were non-US helicopters...
You are right of course. Usually US designed equipment is the best option. In the case of 5th gen fighters you really only have the F-35. There wasn't anything else worth considering. Still the case in fact. However things might change.
The USAF is currently working on NGAD. This is an F-22 replacement and although there are elements of the program we might be interested in, it really doesn't address our needs. For long range strike role the USAF will be acquiring the B-21. It has already been rejected by Australia. This isn't to say that we won't change our minds but for the time being at least it is off the table. The USN F/A-XX program, like the NGAD, is having funding issues and there isn't a lot of info about it. The navy wants to start replacing its rapidly aging F-18 fleet and to be honest time seems to be running out. I wouldn't be surprised if it simply ended up buying more F-35s.
On the subject of F-35s they will remain in production until the mid 2040s and as things stand it may be the only US aircraft the RAAF will consider when replacing its Rhinos and Growlers.
Enter GCAP. Still in its early stages but on paper this aircraft would seem to be closer to what the RAAF is looking for than the F-35, NGAD, F/A-xx or the B-21. We are still looking at a proper replacement for the F-111. The Rhinos were only supposed to be interim replacements for these aircraft. Going into the 2030s the choice may come down to the F-35 vs a better suited GCAP solution. What may decide it in the end is whether it is in our best interest to cultivate regional alliances, as opposed to relying almost solely on the US.
Anyway all speculation. The F-35 may yet be the only show in town when it comes to replacing our 4th gen fighters.