The argument that not everyone has a transponder, and not all transponders work is quite valid, this doesn't reduce the obligations to meet all other communications and operational procedures, including the basic rules of the air, but it helps, a lot.
Don't get me wrong, I'm all for increased fitment of ACAS and cheap compatible transmitters that make small aircraft visible to these systems. However my point was that it can create laziness in looking for the threats and therefore is not a magic bullet that will cure mid air collisions. I do agree it will help in a lot of circumstance as a fall back to eyes and ears.
I think this particular incident it would probably not have helped as it sounds like both knew each other were there, had a plan, and it fell apart in a way things got overwhelming very fast. ACAS is generally inhibited when low to prevent spurious warnings or directions when terra firma is a much larger threat.
As for US/Euro/Botswanan airspace helping here, I really struggle to understand how any OCTA airspace model would have stopped this.
My view on the high rate of MAC in Australia is more along the lines of large amounts of nothingness, then suddenly arriving somewhere with lots of stuff. Most places you go in Australia are empty and you could fly with your eyes/ears shut without hitting anything, terrain, other planes etc... Then you suddenly dump yourself into an intense traffic situation on a weekend or good weather day and you are not used to practicing those separating skills. If you fly in the US or Canada you will see, hear and deal with much more traffic at much greater frequency. Dealing with traffic is a learned skill that is honed with practice and experience. It requires attention like an instrument scan or approach/landing proficiency.