Apologies in advance if this might be better suited to another thread currently running, but personally, I like Ben Sandilands' suggestion: Just cut Sydney loose & leave it to fend for itself.
Sydney Airport? Just cut Sydney loose and get on with life | Plane Talking
After more years than I care to remember, the NSW government (in whatever guise it happens to be at the time) is still no closer to showing the balls to commit to a second airport than it was before. In the meantime, Melbourne's population/economy/infrastructure is growing - with forecasts that it will actually eclipse Sydney as the nation's largest city in the next twenty years - in fact, the rest of Australia is growing, & in the meantime Sydney will end up becoming irrelevant.