If it is an "engineer-business" decision, then why not leave the decision to those directly affected, i.e. the airlines?
But to put your idea in a more logical wrapping: if you have to make decisions, you need data - statistically reasonably sound data. I repeat: there has been no concerted effort by the EU to collect and evaluate such data. Instead, the authorities are flying blind without instruments.
We have data; government aircraft from several nations, including the UK have tested the air quality and stated that its is not good enough for aircraft to fly. You choose to ignore that, as well as ignoring the advice of engineers, engine manufactures and the experience of the many aircraft that have flow through similar concentrations of ash plumes.
Why not allow airlines to make the decision - well, its very obvious from the industries reaction that they will fly no matter what and use the "Gee-whizz who would have though that would happen" excuse to demand a bailout.