I dont understand why Airservices has a say at all in a CASA exemption. CASA is the regulator for the airports, airspace, pilots, controllers, operators, and ASA.
You can get exemptions in other areas as well, like a SAT phone instead of HF, a ADSB MEL which allows flight in the airspace for days.
My guess is that the Industry (big end of town) and Airservices, through ASTRA have looked at the cost of installing additional radar capability on the inside of the J curve and decided that ADSB provided the safest and lowest cost option to provide this additional (safety, efficiency and capacity) surveillance.
The difference between radar and ADSB is more like fixed line broadband and WIFI. ADSB is still in the early adoption phase, and we have not even tried to expand on its capabilities. It is the way of the future.
Eventually it will do more than just ATC. Once the network is in place, it can expanded to deliver a two way datalink service to every aircraft. The datalink for example could allow the transmission of the latest metars for the closest airports, and differential precision updates so aircraft can fly GLS approaches anywhere (there is already a low rate ILS like system based upon transponders called TLS).
Go back twenty years we used to take a photo on film, get it developed, and then talk about with our friends. Today we take a photo with our phone, and whatapp it away.
I think we will still need a primary radar around busy airspace to deal with aircraft failures which would prevent the transmission of ADSB, but the enroute SSR technology wise must be looking like a Motorola analogue brick phone compared to ADSB.
I think the eventual capabilities of the ADSB network have not even been thought of yet.