I think MacBank is smart enough to leave the Australian taxpayer to pay for the cost of the development of a second international airport for Sydney.
You have an odd definition of "soon".
The legal battles on 'The Castle' grounds as well as environmental impact challenges and protests alone will take years and millions to sort out. Changes of government will result in 'go slows', depending on whose access to the trough depends on winning seats in electorates around the existing airport and the proposed new airport. The actual work to build the airport, if it goes ahead, will be the subject of a procurement and contract management process that will be stuffed up by an inept bureaucracy.
Meanwhile MacBank will continue to milk the international joke - whoops, I mean monopoly - whoops I mean privileged asset - that is KSA dry (while paying no corporate tax, of course).
Once all the development work has been paid for by the taxpayer, some bright government will come up with the idea of privatising the airport, thus enabling more millionaires to vacuum money out of the taxpayer. Maybe MacBank will bid for it and win, then shut down KSA ...