Back in the Old Days, 13 / 31 & 17/35 each had 3 runways, plus 04/22 plus an East / West grass strip parallel to Centre Dandenong Rd where the industrial estate is now.
In the " Old Days" there was also a greater diversity of aircraft, some without radio's and no transponders. You probably only have to go back 20 years to have found homebuilts, Tiger Moths, modern era light aircraft, jets and RPT all co-exiting satisfactorily.
I can also recall doing circuits where there were effectively 2 concurrent circuits being flown - inner & outer.
Moorabbin is not the busiest, most complex or most difficult airport in the world. Its simpler than it used to be with less traffic that it has handled in the past. I don't believe there are problems with the facilities, diligence of the controllers or pilots. But, the system and level of resourcing has changed frequently.