I am interested in the question you pose as well... Especially to understand why, if there is an "agility explosion", why trialling does not reflect the trend.
Hopefully I am not misrepresenting the other states when I say, I have heard that there is so much choice in NSW so I haven't got the impression of wait lists and club shortages. The concept of wait lists in VIC sounds very strange (albeit the reality) when I have spoken about it with friends. In Brisbane, the only wait lists I have experienced is when I have signed up for a foundation class while the current class is in progress so I have had to wait for the next one to commence. Not everyone intends to trial so what I have observed are those that do the foundation class and don't continue training (much like enrolling into a one-off puppy class), then those that continue to train just for fun with their dogs (maybe half of the class), and then those who train with the objective of trialling (maybe half of those again)... There are plenty of experienced handlers who do their own training (eg. at home).