I have read several references on this Forum to the fact the Qantas actually try to encourage the purchase of J class over I and even D.

They may even have a policy that they feel it's better to let several J seats go empty than sell them at a discount. The reason is that such a practice would encourage people who want to travel in J class but not wish to pay full rate for it to simply wait until I class becomes available.

It is very much the same with reward seats and upgrades. It is a frequent occurrence that front cabin seats go empty when there are plenty down the back with the points and desire to be upgraded.