I don't think it's a fait accompli that WSI will be much cheaper. It hasn't been developed as a cheaper or lower cost airport, rather it's been developed to add capacity. Nothing about its design and building has been focused on being cheaper. But point taken that it should have a cost advantage because SYD's costs are just that high. Nevertheless, QF/JQ own or have long leases on various key infrastructure at SYD that when combined with their economies of scale when negotiating with SYD does give them a substantial cost advantage that they won't have at WSI.
I don't think we should discount the importance of network effects and feed into JQ's international ops at SYD, especially for routes like ICN and RAR while others like DPS or HKT might be just fine without it. Honestly, I just don't think that we'll see any QF group international at WSI for a while unless the route is specific to the local catchment.