AA is not replacing QF on MEL-LAX. QF currently operate a daily 388 flight and 2x week 789. While not officially announced, many are saying AA will replace the 2x 789 service with 3-5 x weekly flight.
Well going through the current QF 789 rotations, and based on previous QF announcements, you currently have:
4 x 789 doing LHR-PER-MEL-LAX/SFO - LHR is daily and LAX started off 6 pw before they mixed SFO in;
4 x 789 doing BNE-LAX-JFK along with various services to Hong Kong.
So from a 789 fleet perspective if QF drops the MEL-LAX on the 789 then they'll need to be able to rotate the 789's to position them back to MEL to do MEL-PER-LHR. So that implies either a BNE-MEL domestic sector for the 789 for positioning on days SFO-MEL doesn't operate or that they will swap aircraft around in SFO. It also means the 789 comes off of Hong Kong so QF basically has 2 789's to cover off BNE-SFO and BNE-ORD. Do the maths on what the availability is for those along with any slackness in the 3 789's doing BNE-LAX-JFK and positioning requirements for MEL-PER-LHR that's how many potential services you have for the new BNE US routes. I'd say you've got a maximum or 7 to 8 frequencies per week available for them and that you'll likely see 3 per week to SFO. So that leaves probably 3 or 4 frequencies per week to ORD.
Qantas is stretching the legs of their 787-9 fleet to the max, sounds like the time is here to excercise some more options for another 6 to 12 planes. What is going to replace he 744 to Santiago and Johannesburg after the 744 retires. Sydney to Haneda is also in need of a 744 replacement. The current answer seems to be the 787-9 unless Qantas orders something else.
I've maintained for a while now that the next 787 order we see from Qantas will be after the QF/AA JBA goes through. QF have another 6 789's on order and coming into the fleet to allow for the retirement of the 744. How are they backfilling capacity? On JNB they were going to deploy an A330 from PER so you'd have daily SYD on the 789 and the reduction in capacity handled by an A330 service from PER. (Which fell over thanks to the fee dispute between PER and QF) That's still the likeliest outcome for JNB.
On HND, QF have already started MEL and BNE to NRT and re-instated SYD-KIX so moving SYD - HND to a 789 or an A330 and doing supplementary services to SYD-NRT, or having JQ do them, would seem the likeliest outcome to make up for an loss of capacity.
The real question is on SCL but I think that has been partially answered by LATAM starting MEL-SCL service. So QF pulling back a bit of capacity on SYD-SCL while maintaining frequency is likely not a bad thing and with the ability to fly to GRU/GIG part of the project sunrise brief medium / long term 787's flying to SCL won't be an issue because you'll have a 2nd entry point to South America.