Many good comments here, but..
i was infected with the flying bug, about 1963, as a 5 year old at the opening of the Bunbury, W,A. airstrip in 1963.
Flying displays, and skydivers.
I can still see the parachutist who landed off the premises walking past me, in his white overalls, with bundles of canopy in his arms .
And the aircraft!!!!!!!!
and the aircraft.
So, I scratched the skydiving itch many yesrs ago, then discovered the AUF, In about 1990.
Quicksilvers were pretty flash back then, along with Thrusters, Drifters, and many more.
Along comes marriage, mortgages, many years later, 2018, start learning to fly RAAus style.
to buy an aircraft, yes there are Drifters, Thrusters and Quicksilvers still available, but if I am going to pay hangarage for one of those, why not buy a much more capable aircraft that will not cost any more to keep?
And That may be the answer to the question.
For those that want to fly “basic”, rag and tube types, I do not believe there is any impediment. But now, aircraft capable of 90+ knots are affordable to many. So why is there a problem?
yes, numbers of aircraft for sale are down, but there are still aircraft for sale at less than $15,000.
today a Skyfox came on the market and was sold within 12 hours. Good luck to The buyer.
Do I spend about $15,000 on a single seat Sapphire, a lot of fun I admit, or wait and spend $30,000 on a side by side 2 seater...