tdracer : thanks you for your views, I am definitely not a Freight aircraft expert but note the following from an airline I know well :
Lufthansa is not very happy with the performance of their 747-8.
On routes where both types are operated in parallel ( like FRA-SIN) there is a huge difference in passengers booking preferences for the A380. I believe SQ has the same experience. ( but OK, passengers preferences are probably not the major deciding factor)
Finally Lufthansa Cargo replaced it 747Fs by 777-F
.