So usually once a year (maybe twice) I fly from New York to Perth. By distance this is one of the longest journeys. Perth is a few hundred miles from the antipode of NYC. I have the "advantage" that I can fly in pretty much any direction. The two major options for a single stop are via the Middle East or over the Arctic to Hong Kong. There might be other options through say South Korea or Japan but I don't think any airlines fly direct from Perth to these. Oh and I think China Southern can do it via China.
Anyway. this amounts to roughly 24 hours (total) in aircraft time. I'm 6'2" and wider than a coach seat so this would be torture. I am fortunate enough to be able to afford a premium cabin so it's one of the things I choose to spend money on. This might mean paying 3x as much for a flight. I don't care.
Anyway, a big problem with even longer haul flights is the added cost of carrying the fuel you need for those last few thousand miles. I think Qantas estimated the per passenger cost of LHR to PER is about 30-40% higher than making a stop.
SYD-JFK is ~10% longer than LHR-PER, which will make the cost what? 60-70% more than going via LAX or DFW.
The problem of course too is if your final destination from Sydney isn't London or New York then you need to make a connection anyway.
So this is where say the ME3 carriers shine: ultra long routes with a single stop and a lot of options for city pairings.
Anyway, I'm sure someone will want these flights. I don't expect any direct NYC to PER flights anytime soon however. :)