There is another reason QF's fleet could be written down - maintenance and modifications. There is a huge difference in value between a clapped out aircraft that has been minimally (but legally) maintained and one that has had heaps of modifications during its life to improve its serviceability and/or performance.
Ansett in the old days had this down to an art form, although this was as much about hiding profits as anything (ie doing a mod that improved capital value, but of course charging it as an "expense")
A case in point were the DC-9's that Ansett sold to Evergreen. Uncles Peter and Rupert did rather well out of that.
Woops! I forgot to add the necessary quote from Geoff Dixon "If our aircraft are written down then we will have to cut costs further."