I have no idea what happened and so dont want to comment other than to say it is of course sad news.
In general however I do feel it is worth reiterating that all the evidence suggests "see and avoid" doesnt work - or at best works poorly. It is one of those myths that nearly everyone believes in so we just accept it must be so.
Secondly if you can it re- enforces the need to get high en route. Yesterday at FL50 the viz was unlimited giving you the very best chance to see another aircraft. Moreover looking down it was reasonably easy to spot other aircraft whereas in the clag it would have been almost impossible.
Is it just me or do these sort of weather days seem to be a lot more common that twenty years ago?