Obviously ultra long-haul flights are less fuel efficient, because transporting the fuel uses more fuel. But in the end, it will come down to money, and airlines will probably avoid it unless people are willing to pay a premium to avoid the stop. My guess is that ultra long-haul will remain rare, and limited to the largest markets. Perhaps JFK-SYD might be feasible a few times a week, but we aren't going to suddenly start seeing JFK-MEL or PHL-SYD or anything like that.