I for one don't see the point of having two airports in Sydney with curfews. You might as well just stick with YSSY if that's the case.
Pretty simple really, YSSY is already crowded. No point adding additional runways which would just increase the overcrowded airways and ground facilities.
Richmond would add a lot of additional capacity and potential capacity, for minimal cost - including building the rail links.
No brainer really.
Best part of it is we stop getting sucked dry by Macquaire, for everything from parking to passenger charges!