If QF does replace 744 with 789 om AUS-USA i wonder how many routes to AA hubs get 1x QF and 1x AA 789s to up frequency, vs just QF change or double QF.
Depends on exactly what you mean.
SYD/MEL-LAX will NOT be B789, already A380 and will be for at least the next decade.
SYD-DFW, most likely will stay A380, MEL-DFW will start with B789, but my guess is that within a few years it will need an A380 or two B789s, the A380 should win that one.
B789 could be used to pioneer an extra frequency from both SYD/MEL, but that's a problem if you have robust growth.
BNE-LAX, could go B789 but I understand that the decrease in capacity could be a problem, possibly room for AA IF they are willing to make the investment.
SYD-SFO same as BNE-LAX, although SFO seems unlikely for AA.
SYD-YVR definitely B789
This leaves the question of what to do about JFK. The current B744 is the BNE-LAX aircraft, if that goes B789 then JFK goes A380 or B789 both choices have problems.
To comment on your question specifically: maybe LAX-MEL could use an AA B77W or B787 rather than the second flight, which not currently daily, going B789. IF BNE-LAX goes B789, then an AA B787 may make sense. Beyond these two possibilities I don't see much attraction for AA. To be honest I really doubt the profitability of AA LAX-SYD/AKL flights in the long run. Will it be 3rd time's the charm for AA? Who knows, watch this space,