No such standards exist for volcanic ash, no testing has been done to determine at what level the ash becomes a threat, and no reliable method exists to measure the ash threat real time. Faced with these known unknowns, there is no other choice but to be conservative.
The KLM 747 event came dangerously close to being catastrophic - likely resulting in a large loss of life. The investigation of the engines suggests that the reason they were able to get them restarted because the heat cycles from the repeated failed restart attempts caused some of the glass coatings in the turbine to breakup and be shed, allowing for a successful restart
Tdracer
There are a couple of points to consider? firstly the major events of intrusion into volcanic ash where the engines nearly failed completely were into dense volcanic ash clouds not prolonged flight in ash concentrates which are so low as to not be visible in daylight and out of moisture clouds.
Secondly while I appreciate prolonged flight into low density level ash could reduce engine life that then becomes a commercial consideration and not so much a safety consideration!
Where we do not appear to have answers is the density levels which will cause instant failure in the flight and hence a perceived threat to safety.
In most aviation safety regulating demonstrated safety holes are plugged this is perceived threat in the sense that there has been no loss of life throughout the course of aviation history.
I appreciate that had the engines not started that may not be the case now.
its a bit like regulating a reduced speed limit on a known stretch of road with a high fatality rate and regulating a speed limit where there have been no fatalities because the powers that be consider a set of bends may cause an accident in the future.
Both have merit but in this case we do not really have the data and hence have probably been too cautious in picking ash levels out of a hat rather than using proven scientific knowledge and fact.
Also remember the previous event was driven by the media, political pressure and public fear and politicians not scientists danced to that fear.