Won't see it in our lifetime, bad idea.
Plenty of life left in YSSY, just remove all the political hurdles like curfew and movement caps.
Redevelop the terminals, fill in the area between the N/S Runways and move the freight and maintenance hangers out there with access from Foreshore Drive, extend T1 around to the current freight apron and demolish the QANTAS hangars to make way for an expanded domestic terminal on the north side.
There's no money in the budget for a second sydney airport if you believe the Lieberal spin machine and who is going to invest privately? Don't forget also that Macquarie have first right of refusal as well.