Such a waste and condolences go out to all the families concerned - RIP
It is accidents like this and others that should mandate all Light aircraft to have at least a mode A transponder fitted. I am not saying it will prevent accidents on its own, nor will it ever excuse maintaining the best lookout in open airspace that you can, but it would at least show you up to ATC.
As a Pilot who regularly flies in that airspace it is amazing how many primary only contacts get called to me on a traffic service from ATC that always seem to be single piston/gliders/microlights. I am fortunate that i fly an aircraft with TCAS - and it has saved me on more than one occasion in that airspace, but it only works if you are transponder equipped - remember it saves ME and YOU.
During my early career i regularly flew an aircraft without a transponder and without an ATC service - not now, having a transponder and using it will save your life.
I realise it costs to adopt these measures and the GA community has resisted any mandated regulation - but think of what it would potentially save..
Per Ardua Ad Astra