B Fraser, your link leads to a document which seems to counter your own argument!
The most alarming of these encounters, involving temporary engine failure, have naturally received disproportionate publicity. But the fact that only 10 such incidents have been recorded in 26 years gives an indication of their rarity. Although few quantitative details are available, it is clear that the most serious incidents have all occurred in the 'soup' relatively near the volcano.
A well documented example occured on 15 December 1989 when a KLM Boeing 747-400 encountered flameout of its engines due to ash when flying near an eruption of Mount Redoubt in Alaska[16]. The damage was estimated to have cost some US $80m to repair. There was 80kg of ash in each turbine and the calculated ash density was 2g/m3. Given the air ingestion rate above and if all the ingested ash was retained, then 62/3 minutes exposure would have been sufficient to collect 80kg. However for the limiting density on which the no fly ban was initiated, 4mg/m3, only 160g would have been ingested, emphasising the huge range of densities from the flame out magnitude down to the level of the flying ban.
But safety is not the only consideration. In 2007, it was stated that[17] “the economic cost of volcanic ash to international civil aviation is staggering. This involves numerous complete engine changes, engine overhauls, airframe refurbishing, window re-polishing and/or replacement and pitot-static system repair, etc., and the inevitable loss of revenue due to aircraft down-time while the foregoing is accomplished. Delays to aircraft and their rerouting around volcanic ash has caused considerable expense to airlines operating in regions prone to volcanic eruptions. Also to be included is the cost of volcanic ash clearance from airports and the damage caused to equipment and buildings on the ground. Various estimates have been made, most citing costs to aviation well in excess of $250 million since 1982”.
These figures are small compared with recent estimates of the cost of the 2010 disruption, "EU Transport Commissioner Siim Kallas said the economic impact of the weeklong crisis had caused losses of estimated between a1.5–a2.5 billion"[18].
That paragraph is quite telling.
The disproportionate reaction of the authorities in banning all movements by air caused far more disruption and loss than the volcano would have if everything had been left as it was.
Don't forget,
all flying was stopped, i.e. recreational and other forms of private aviation, not just that of airliners, which, it could be argued, are most likely to be badly affected.