My own speculation.
There are currently 18 A321 LR's and 36 A321 XLR's on order (firm) for the Qantas Group. The majority of these will go to JQ to replace the B787-8 on Asian routes - the B787's will transfer to mainline as replacements for the older A330's which are coming up for retirement but there has been no announcements about ordering more A330's or looking for a replacement. It's a good way to get mainline to pay for the heavy maintenance checks on the B787's.
At least 15 of those A321 orders will probably go to mainline to replace the 15 B737-800's that were ordered after the Ansett collapse, these are the oldest in the domestic fleet. This will give mainline an aircraft sized between the B737-800 & the A330-200 - they have been missing the B767 ever since they retired that fleet. Once the A321's have established themselves in mainline operations, the rest of the B737 fleet will be replaced by A320's & A321's over a 15 year period.
Mainline Domestic will eventually be an all Airbus operation and International will be all Boeing (after 2027) as the B777-8/9's will be ordered to replace the A380's and B787-10's will be ordered to replace the remaining A330's (the B747's have already been replaced - APPARENTLY).
There will be many that disagree with this but, time will tell.