GA is under threat from two sides - over regulation by CASA, and being squeezed out by airport owners.
Now while Dick Smith may a different way of expressing his opinions and feelings, I reckon he's fairly close to the truth. I started flying in October 1977 at Parafield when the grass parking area to the East of the Eastern apron and South of the Southern apron was chock-a-block with light aircraft - I'd guess about 100-120, all hardstands were full, and flying schools were busy.
When I became an aircraft owner in August 1996 there were a few parked on grass off the aprons - my guess would be 20-30 at the most, with the hardstand to the South full but to the East starting to thin. In July 2016 there are (generally) no aircraft on the grass,with plenty of unoccupied hardstand restricted to local operators.
Most secondary airports are being encroached upon by shopping centres and outlet malls, with airport owners relishing the day when they can say "sorry, there's not enough business to keep it open", and there'll be champagne corks popping as the sites are rezoned residential.
So Dick, I think you're right on the money, it's only a matter of time.