Rather, we cannot agree on what constitutes "standard ash" against which to test or regulate. There's no such thing as a bird-proof engine, either, if the ambient bird density is high enough...
There is also no such thing as a standard bird either.
The environmental encounter regulations are based on probability of encounter. The probability of encounter considers historical experience. Thats why the Feds want all bird strikes reported so as to approximate the sizes, quanities per engine and results. The encounter stuff obviously contains all possibilities of avoidance (both by the birds as well as man). Additionally it keeps track of nature changing by virtue of things like the canada goose etc.
The results of the encounter also consider the good and bad point of changing technologies in the product (high-bypass, wide chord fan blades, FADECs etc. etc.)
When it comes to volcanic ash and super cooled droplets the data to base a standard test on need to consider how often you expect to encounter any given size and density per minute as well as available technolgy within the aircraft to withstand the encounter.
The prioritization for answering the questions about designing to a known standard would also consider what level of standard provides a freedom from risk of an serious result to something ten to 100 times better than the overall risk we are flying within today.
So it seems that since the average operations today in a volcanic ash enviornment have not exceeded the risk floor of 10 to 100 times better than the average risk from all causes, that maybe we should be chasing after some bigger fish like super-cooled droplets in both avoidance and capability.
BTW, cost doesn't enter into setting min safety standards but of course does enter into everything else in how to run a business.