The A350 business case was made at a time when it was flying and the 777X programme was stalled with engine problems and Boeing having to divert resources into getting the 737Max re certified.
The A350 was chosen at the time because there were airframes available within the chosen timeline and an aircraft already flying carries far less risk of delay than one still under development. The choice was made despite the 777X having superior performance and mission capability.
Fast forward a year from that decision, if QF execs still have an appetite for ultra long haul operations (which I believe will have a lot of demand in a post covid world) then the decision is very different. The 777X has flown, engine problems mostly sorted, delivery dates less likely to blow out and many customers looking to cancel orders hence early delivery slots may open up for QF.