You will need to convince RA-Aus to fit transponders to thousands of aircraft and the GA aircraft that don't have them too.
There is nothing to stop a light aircraft bobbing along clear of cloud below 3000 without transponder and an IFR jobbie punching down through it. Luckily we aren't heavy airspace users in those places without primary radar otherwise this would be a far more frequent occurrence.
What is the solution? ADSB???
There will always be inaccurate or faulty transponders or those without decent radio skills and position awareness.