This Discussion is going around in circles...
The reason why is because certain people hang their hat on 1. ADSB 'In" not being ready and installed in aircraft. and 2. The reliance on GPS relating to the switchover from SSR to ADSB, and the risk of 'turning off' the GPS network.
Firstly ADSB 'In' is irrelevant in the argument of whether to use ADSB as a replacement for SSR radar. With ADSB 'Out' the changeover is transparent to all operators, as some will still have a TCAS display and some won't but other than ATC being able to run traffic closer to each other in remote (currently non-radar procedural) areas, the operator will not notice any difference. The big change will be from the ATC side of the fence, as they will be able to have widespread surveillance across Australia instead of the current situation where the majority of Australia is currently non-radar. If in the future ADSB 'In' becomes available and widespread then yes it will offer further advantages to operators by giving increased Situational Awareness to operators, but the current lack of this is irrelevant to this current discussion, so please stay on topic and stay relevant.
Secondly as has been mentioned earlier, the Primary radars will remain and I wouls suggest that given there is a lot of overlap of radar coverage on the East Coast, there will be areas where they can switch of a few radar heads (delivering big savings) but not effect the effective SSR coverage to any detrimental degree. What it would mean is some of the current redundancy in the system would be lost, but this would be acceptable given that SSR would now be the backup redundant system anyway. Also if anyone really thinks that the GPS network will ever be switched off or intentionally degraded they are dreaming. That is like suggesting that due to Terrorists using the internet to communicate, the internet will be turned off.
If it really does become apparent that terrorists are using GPS to guide their unmanned bombs (I highly doubt that and think the hijacking of a plane still is the easiest way to achieve the bad guys ends), then I am of the understanding that an error can be intentionally introduced into the GPS network. Now even if 1 Nautical Mile was introduced into the network and we had to run 7NM between all ADSB traffic it still beats 30NM RNAV standards that we run currently in non radar airspace.
I will say again that an ADSB ground unit is extremely low cost, runs on a tiny amount of electricity (easily supported by solar panels) and has no moving parts so minimal maintenance needed. This means that for the price of one radar head and the related calibration and maintenance process, ASA could pay for the installation of an entire network of ADSB units in remote areas. To use a Dickism, "I'm sure you will all agree" that this is a positive outcome for all customers.