I know it is in the conditions that you can only use the credit for a higher priced fare, but that is really a pretty savage condition that takes advantage of Qantas's market power relative to the low power of the consumer. Also, I think most people are not aware of what this really means until it is time for them to try to use the credit. I really wonder if this is enough to amount to unconscionable contract (I am not a lawyer, just feels like it to me as a lay person). And yes I know that people have the choice (sometimes) to use a different airline.
Pre COVID, I purchased a very expensive Business Class ticket to the USA for a trip that was supposed to be a combined work + holiday trip. The work part got cancelled at last minute, and for various family reasons associated with ill health of elderly parents, I was unable to travel for the leisure part, so I put it into credit.
When I went to use the credit, air fares had fallen so much that the credit was more than the most expensive business class ticket to anywhere in the world. So my only option at that time to use the credit of over $10,000 was to upgrade to First at a further very high cost. I said that therefore I would just have to cancel the credit even though I would not get a refund.
I was extremely lucky that I got a sympathetic agent who said that was ridiculous outcome, and they would just extend the credit by another year for me. It probably helped that at the time I was a WP and well on my way to becoming a P1, which I did achieve shortly after the phone call. Anyway, another year passed and I was still unable to fly before the credit expired, but fares had crept up a bit, new routes had opened up, and so on the last day of credit, I booked for 12 months out, paid only a few hundred dollars for the new fare and ultimately had a wonderful holiday trip 3 years after I originally intended to go.
That was one good experience, and it certainly taught me a lesson. If I think there is any risk at all that I will need to cancel a ticket, I now buy a fully refundable ticket. Then if something good comes up, I can buy that and cancel for a refund. It does mean that there is a timing difference in the cash flow, but fortunately, I can sustain that.