Perhaps QF would be better taking some of the 787's planned for JQ & use them on for example, MEL/LAX, BNE/LAX, SYD/LAX on days when larger aircraft can't be justified
JQ's 788s don't have the legs to go trans-Pacific without restrictions. You would need 789s for that.
You can't have spare fleet waiting around to be used on days when bookings don't justify a larger plane. The economics don't allow it. You either permanently downsize the route or stimulate demand to fill up the larger plane.