I'm concerned here about the lack of rational common sense thinking about a Second Sydney airport.
The trouble is that you are thinking logically and not politically.
The reality is that a Sydney second airport will only be built when the politics are right. You can forget all the BS about gridlock, congestion, etc. because the people who get things done in NSW don't care.
A second Sydeny airport will be built when:
1. The rich cats in the Sydney property development and merchant banking community work out how to make billions out of the project - perhaps by building One new airport and redeveloping Mascot as a suburb.
2. The Union heirarchies work out how to make $$$$ for themselves along with the groups in (1) and forward their political careers at the same time.
3. The Sydney "push" (groups 1 & 2) work out how to screw Melbourne and Brisbane for industry development by maximising the difficulty of International passengers transiting to Melbourne and Brisbane. For example, put all domestic traffic at the new airport and leave international at Mascot.
4. All Three groups work out how to get Australians as a whole to pay for Sydneys new airport.
Frankly, I hope it never gets built and Sydney chokes on its own traffic. Then perhaps Melbourne and Brisbane will get more attention and more direct flights to international destinations.