Plenty of life left in YSSY, just remove all the political hurdles like curfew and movement caps.
And which politician is going to sign up for that idea?
Besides that the non aviation infrastructure at SYD is severely lacking for an airport its size. The train is prohibitively expensive and the roads don't cope at the international or domestic.
Even if you built new terminals and carparks there is no means of reliably transporting people between them. You need some sort of monorail like DFW or LGW.
A properly built 24 hour CAT III airport with a decent train service and road network is what is required....................just like the rest of the civilised world.