Leadsled,
Have you been to GA website and read the "..supportable reason for what is going on." ?
Here let me google that for you; www.ga.gov.au/scientific-topics/positioning-navigation/positioning-for-the-future.
The Australian Govt body charged with advising on national positioning infrastructure, has identified that there may be a need for a WAAS. As such they are running a trial to see whether that hypothesis is correct.
This trial has nothing to do with aviation, but just so happens that if the correct signal is commissioned then there are existing avionics that can benefit.
The cost proposal for aviation includes funding from the Federal Govt for design, validation and publication of SBAS app at all eligible airfields.
So far as I can see, none if the major costs of SBAS implementation are being borne by the Aviation sector. This is a Federal Govt initiative and so will be funded by all taxpayers (if the cost benefit analysis proves its worth)
You keep asking what has changed? Whats changed is that previously Aviation sector were the only proponent for SBAS in times when no one else knew what SBAS was. Now GA are driving this for a different purpose, Aviation may benefit.
I say may because GA are trialling an SBAS II signal and there are no aviation avionics that can use SBAS II. If SBAS II gets implemented, aviation will get nothing.