peter we
Your problem seems to stem from a refusal to accept that when manufacture's say there is no safe level of ash ingestion, they mean it. The more ash, the higher the probability of damage, there is no 'safe' cut off level that you seek.
This is just semantics.
The manufacturers have not stated that "there is no safe level of ash ingestion"
The manufacturers can not tell you when it is unsafe to fly but can tell you when it is safe to fly. The zone between the two is subject to how you intend to operate in the nether region.
The manufactures can only state that there is no known defined point of safe vs unsafe level and that therefore it is up to the variances of individual operations to establish safe operating procedures. (goes pretty much for other enviromental hazards as well, including birds and ice) Instead they advise a degree of avoidance of concentrations vs procedures to minimize impacts on the safety of a specific flight.
What the operators are doing about costs is their business and not the business of naysayers in forums.