I think there is sufficient demand Mark. DFW is a very large AA hub so many QF code share connections are available. There are probably very few routes that the A380 would be justified on over a 15hr sector, but this is one of them.
Air NZ makes a lot of money due to their monopoly, non stop routes. This is one of those for QF. SYD - DXB - LHR, sadly, is not.