I have patiently explained about five times on this thread exactly what the problem with ash ingestion is :- extremely costly damage to engines
Sunfish
On what basis are you making such sweeping assumptions? Do you think the Airlines who are prepared to fly in low levels of ash pollution are also so stupid that they have not weighed up the potential engine damage costs (if infact they do get significant engine damage in their fleets) against the losses they would incur by as you put it "occasional airspace closures"?
They have already operated in low level ash and to date I have not heard of significant damage. I am sure if there was they would be the first to ground their own fleets on a cost basis.
I do not know what your background is to make such assumptions over what the airlines have decided to do with their own level of operational knowledge.
I do not know long term what the costs will or will not be but I guess neither do you. It will not be our problem if you are right but I am sure the airlines will have weighed up all the risks and costs when deciding to operate in low level density ash.
Pace