Travel plans change.
Where are you based? If in SYD/BNE/MEL that return Qantas flight is going to cost you ~$190 perhaps less. It gives you Gold status for at least a year.
It also puts you a little higher in the pecking order when it comes to upgrades and op-ups.
You need four eligible Qantas flights as well as 700 SC's to earn Gold. The OP has only one eligible QF flight as highlighted in the first paragraph of their post:
The OP mentioned they have done 'one Qantas return as a qualifying flight' so I take that to mean 2 flights.