It's a while since I used to read light aircraft incident reports regularly, but this old head instantly thought of at least two other mid-airs in VMC within 100 miles in just the South East corner of the country ... a classic climbing and descending at Redhill (one a Cherokee I think), and two Robins over Kent (one on a training sortie out of Rochester / one routing cross channel from Ockham way). There have surely been others in other parts of the country over the past decade or two, and I am not sure we can still call it a "rare" or "unusual" occurrence, or express surprise when it happens.
We all know that 'looking out' and 'scanning' is mandatory but let's face it, at typical 200 to 250 kt closing speeds in 2007, and with typical 'eyes in' workloads of single pilot navigation and instrument training, and varying standards of Class II sight, what we are really gambling our lives on when eyes are in, is simple laws of probability based on unknown traffic densities unless at least RIS has been obtained.
Maybe it is indeed time to ensure that RIS is mandatory in some cramped corners i.e. upgrade the airspace, rather than to be allowed to continue to fool ourselves that like old times we can live without it ...
I think pilots deep down do know the risk we take is there and could be offset with investment, but our thoughts now are of course with the families of the two people who died.