Dick,
ADSB is the future. Australian aviation is moving away from fixed route structures to more flexible use of airspace, user preferred route structures. This can only be delivered where the surveillance is either radar or ADSB. Fixed route structures are being phased out.
The transfer from £sd to decimal currency and the transfer from imperial to metric weren't easy, neither will the transfer to ADSB, but it is arguably for the better.
(AIC H24/14)
In order to take full advantage of this modern technology, Airservices Australia is installing additional ADS-B ground stations to enhance coverage and service provision for the lower level operations across continental Australia in addition to that currently available at or above FL290. As more ADS-B ground stations are added, some radars will be decommissioned as they reach their end of life date. Surveillance will be increasingly provided to ATC by ADS-B only. ADS-B fitment also facilitates the cockpit display of nearby traffic based on ADS-B transmissions thus increasing efficiency and safety.
Furthermore, air traffic control separation services are increasingly based on required navigation performance (RNP) standards as specified in CAO 20.91. Increased operational efficiency can be derived through early GNSS fitment allowing RNP derived separation to be applied. In addition, the progressive decommissioning of ground based navigation aids (reducing to the core backup network) will result in an increasing reliance on GNSS-based approaches.
Reducing the procedural separation standards from ICAO approved 10 minutes (lets say 60-100 miles) to ICAO approved 5 NM ADSB separation standard is a quantum leap in terms of airspace capacity, aircraft being able to operate at their preferred levels sooner and for longer, weather diversions, situational awareness.
Having to apply procedural separation standards between ADS-B and non-ADS-B aircraft outside of radar coverage is the crux of the matter. It means a high degree of complexity that voids many of the benefits that ADSB brings, and I believe that this is the reason for the mandate; a mandate gets to the end state sooner and with a shorter period of operating a hybrid system.