What better risk analysis can you have than 50 years of records and millions of flights?
The undisputable facts are that NO ONE has been killed in the whole history of aviation by ASH.
What a very strange thing to come out with. You could use the same logic with Concord; it had a perfect safety record, until it didn't.
Maybe its becuase aircraft have always avoided flying through dust, certainly when they do, in certain concentrations its got very very expensive and dangerous. And VA is not the same from all eruptions, some, like this, are far more damaging.