And really the only advantage the Super Hornets have is that they're based on carriers so they can project power from unexpected vectors. Putting them on land kills this advantages, so why would anyone want to do that unless they have no alternatives.
In Australia's case, it was because Super Hornet allowed us to compensate for our own short-sighted decision-making (re: F-35 project timelines and declining F-111 and F/A-18 readiness) by drawing directly from USN order slots on an active production line.
I think that you're a bit off about China's situation. Right now, they've actually been retiring fighters much faster than the US is. All the Su-27s and Su-30s are gone, the J-7s and J-8s are either gone or reserved for training units, and the PLAN will probably get rid of the J-15s soon even though they're essentially new. The actual number of fighters hasn't increased all that much; they've just been replacing old planes with brand new ones at a very high rate. Right now, the retiring still isn't over yet and I imagine they're going to start on the older J-10s and J-11s.
That's fair. To be honest, I was mostly thinking about warships. I guess the broader point is that it's worth paying attention to what is happening to the oldest and least capable elements of the inventory because, in concert with current production and programs of record, it's a clue to what is envisioned in the medium-term.
Honestly, this is what the Americans should have been doing, so I'm going to call them out for incompetence. They should have gotten rid of their old junk ages ago, but because they were still capable platforms for the US' endless expeditionary wars, they're useless fighting against a foe with a fully modern air force. To date, this only matters in a conflict against China, but that's the fight that the Americans are psyching themselves up for.
The difficulty with that idea is that a USAF divested of its older platforms is a dramatically smaller USAF, because few airframes were inducted in the post-Cold War era and both fifth-generation projects were delayed, over budget, and experienced significant ongoing challenges in terms of operations and maintenance costs and future development paths.
One can appreciate where the basic idea came from. The objective was to bridge between the dominant force structure of the then-present, to the equally dominant VLO force structure of the future. That transition was threatened by the lower budgets of the post-Cold War era but, conversely, the "unipolar moment" allowed USAF to divert resources from near-term procurement to longer-term R&D. The difficulty is that those longer term bets have not paid off to the extent that was envisioned.
2021 paper from the Mitchell Institute goes into the arguments against the acquisition of F-15EX, and those arguments are compelling enough on their own terms. Zooming out a little, though, it's more than a little odd that USAF advocates are at all troubled by services (PLAAF/PLANAF) that are implementing force structure strategies that are explicitly rejected for USAF, i.e. the ongoing production of advanced fourth-generation fighter jets.
One conclusion that may be drawn is that these dichotomies that are presented (investing in advanced fourth-generation aircraft vs. investing in fifth-generation aircraft vs. accelerating R&D for sixth-generation aircraft) only present as such because there is insufficient budgetary, institutional, even political space to pursue those paths simultaneously. Using the American nomenclature for the purposes of comparison, China currently has two advanced fourth-generation combat aircraft (counting J-15/16 as essentially one design and production lineage), two fifth-generation combat aircraft, and at least two developmental sixth-generation combat aircraft (that are clearly complementary, rather than one being an alternative to the other). Orders, budgets and personnel can be shifted around between these projects as required. The American system today simply has fewer moving pieces, and so the choices present with much harder-edged trade-offs between competing priorities.