Agreed. If the government want airlines and people from Greater Sydney (as a whole) to use the airport. rather than legislating, invest in a HKG style rapid, frequent train service into a city terminal where people can also check in for their flights.
Other than that, the airport still has the potential to serve about the population of Adelaide - that being people in the LGA's which are mostly or entirely closer to WSI than SYD. Different to AVV, where in Melbourne itself, there is only 1 LGA with about 300,000 people that is predominantly closer to AVV then MEL, although there's another 300,000 in Geelong/Surf Coast etc.