Given that the GA fleet is ageing, and the average age of aircraft seems to be 40 years, what happened 40 years ago that a whole fleet of aircraft were brought into Aus?
No single answer.
Aircraft were relatively far cheaper to purchase and cost far less to operate and subsidised finance was available. GA aircraft designs were not stifled by modern regulation and most of the development cost, particularly for US built aircraft, was funded my the US military or NASA. Can you imagine the difference in design and construction cost of a 1960s Cessna seat, versus todays 24G resistant high tech safety seats?
Fuel was very cheap. I don't recall the price per gallon but I do recall a crude price of $7 per barrel (159 liters) in the mid to late 1960s.
The cost of flying training was subsidised by Government. In 1961 my local aero club charged £4/10/- ($9) per hour for a Chipmunk, although as a 17 year old I think my wage was somewhere around £8/10/- ($17) per week.
The cost of airline flying was prohibitively expensive and until the 1980s, very few Australians had ever flown in an airline aircraft. The only airfare I remember was the Ansett flying boat service between Hobart and Sydney; in 1952 my parents flew Hobart to Sydney and back and I seem to recall their return air fares for two was around £600 ($1,200) or approximately two thirds the price of a new Holden FX sedan.
GA aircraft are now older and far more expensive to maintain, whilst the very significant reduction in the cost of airline travel has put down ward pressure on the price a GA operator can charge.
In the 1950s, 1960s and early 1970s rural airservices were Government subsidised through the predecessor of the Rural Air Services Scheme (RASS), introduced by the Menzies Government.
And in those days, the DCA inspector was a reasonable, intelligent and rather knowledgeable fellow, a current and competent pilot who probably had the odd DFC or DSO earned in battle, received around 150 flying hours per year at DCA expense and who knew how to sort out problems without fuss around the back of the hangar or over tea and bikkies in the crew room.