It's more expensive to run the A380 on any route compared to the B787-9. But it can also generate a lot more revenue.Presumably they would be blocking quite a few seats on a 388 flying DFW-SYD.
I remember when I attended a QF feedback session last year and there were quite a few C-level executives and other senior staff. One of them made an off the cuff comment about how much more expensive it was for them to run 388s to DFW.
But yes, quite a few blocked seats, but they don't mind blocking many economy seats if it means selling more in first, business and/or premium economy. FWIW, B787-9 also has to block seats at this time of year. On Saturday the B787-9 had to go into BNE for fuel on the way back from DFW.
