You obviously haven't heard of, or have forgotten about, places like Ballina and Mildura, PC. Google them and "ATSB".
There is no risk calculation. It's currently a game of pass-the-risk-parcel.
Could you provide details of the collision-avoidance tech fitted to RPT jets that detects aircraft that don't have transponders?
(Plenty of GA involves certified, transponder-equipped aircraft flown by licensed and medically-certified pilots.)