Similar considerations to the Perth - London non stop with the B787 which was put forward a few months ago. Misery in economy if they use a 10 across configuration. Expensive to operate with the increased fuel burn compared to an enroute top up and increased crewing requirements.
Emirates can do Sydney - New York one stop via Dubai with lower costs involved and can pass the savings on with lower fares.
However Sydney and New York have much higher potential traffic than PER - LON including more of the lucrative business travel market. A flight with First, Business and Premium Economy rather than regular Economy, at a reasonable cost over a connecting flight could be an attractive proposition.