Ultralights and wishtobeflying – you're merely making Sunfish's point (perhaps inadvertently).
The reduction in numbers of organisations to which AMROBA refers could be caused by any number of factors, none of which necessarily means "less aircraft are flying". I suspect that there are actually more aircraft flying today in Australia than there where in 1990, but fewer of them require maintenance from the organisations which AMROBA represents. And I suspect that many of those aircraft don't operate in and out of GAAP aerodromes, their pilots preferring instead to avoid the charges.
However (and I think this is Sunfish's point) my suspicions are all well and good, but cold hard facts might be a little more useful in convincing government that GA is rooned, if so why, and if its just evolution and the market, why GA shouldn't be exposed to the same market and evolutionary forces as any other industry.
When the car and train were invented, the number of organisations that serviced horses and carts declined.
When electric and diesel trains were invented, the number of organisations that serviced steam locomotives declined.
The number of organisations that fix TVs/videos/dvds/radios is in decline, because the march of technology means that they are becoming throw away items: it's cheaper and quicker to buy a new one, rather than pay someone to fix it.
Just what do you expect the government can and should do to maintain the use of and infrastructure around 1950/60s aviation technology?