Maybe not. Given Australia's distance from other populated countries (okay, the US), the added distance to travel to Melbourne on a single flight may be non-trivial. Additionally, the amount of travel on a given day to Australia might not be enough for an airline to dedicate large 747 or A380 routes to multiple airports; people flying 16 hours over the Pacific might not expect a nonstop flight regardless. Hence, Sydney became a defacto hub for much of Australia's air travel. And once you're already paying one airline to get you there, you might as well take their connection to Melbourne, rather than pay another vendor, collect your baggage, find the train station, etc.
Just this past May I flew from SFO to MEL. My options on United were SFO-SYD-MEL or SFO-LAX-MEL. I actually preferred the Sydney connection because I reasoned a missed connection in Sydney wouldn't put me off my journey by much, but a missed connection at 9pm in LAX would ensure I'd lose at last half a day if not a whole day getting to Australia. Ultimately I went with the SFO-LAX-MEL route for 2 reasons; the Sydney option routed me on Qantas for the Australia "domestic" leg and I didn't get any miles for it, plus the LAX-MEL route was served by a 787 which I deemed preferable for its potential to reduce jet lag (jury's still out on that one).
The advent of smaller planes like the 787 serving MEL from, one would hope, an increasing number of US cities, might take stress off of SYD.
Just this past May I flew from SFO to MEL. My options on United were SFO-SYD-MEL or SFO-LAX-MEL. I actually preferred the Sydney connection because I reasoned a missed connection in Sydney wouldn't put me off my journey by much, but a missed connection at 9pm in LAX would ensure I'd lose at last half a day if not a whole day getting to Australia. Ultimately I went with the SFO-LAX-MEL route for 2 reasons; the Sydney option routed me on Qantas for the Australia "domestic" leg and I didn't get any miles for it, plus the LAX-MEL route was served by a 787 which I deemed preferable for its potential to reduce jet lag (jury's still out on that one).
The advent of smaller planes like the 787 serving MEL from, one would hope, an increasing number of US cities, might take stress off of SYD.