You can't really compare London's 5 airports with Sydney. Completely different geographic situation. From Wiki: The airports serve a total of 14 domestic destinations and 396 international destinations. (vs Sydney's 46 domestic and 43 International). The latter is why there are 5 airports. Its not the 9 million people who live there, its the 200 million who fly through there. I'd hazard a guess none of them are "feeder" airports. You can get to where you want to go from any of them.