Why on Earth would you need, or want, to close Bankstown to use Richmond? Overseas airports often have close proximity to other aerodromes & still manage to function effectively.
For example, in Orlando where I now live, Orlando Int'l (KMCO) has four runways that point straight over the top of Orlando Executive's (KORL) 07/25 & 13/31 runways or over the approach/departure paths. The FAF for the 18R LOC (LLZ) at KMCO is overhead KORL. That's only 6 or 7 nm from the threshold. I've lost track of the number of ILS or LOC backcourse approaches I've done to KORL while KMCO traffic is going over the top on their own approaches.
Not to mention the other airports with instrument approaches within in 20 or so miles eg Kissimmee at 10nm or Sanford Intl at 20. Every one of these aerodromes have ILS, VOR, NDB &/or GPS approaches. And these aren't unusual in the US. There are any number of airports in the US that are close yet handle large numbers of mixed GA & airline traffic.
So, why does Oz seem to need 50nm or 100nm or whatever between airports? I see no reason why YSBK couldn't operate with RIC & YSSY - and Camden. And Hoxton Park.