In aviation, if you don't know, you don't go, which is what has made air travel so safe.
I disagree, that has never been the approach in aviation. What has been the approach is a graduated approach to risk, not a risk free environment. It is abundantly clear that the ash cloud is not having the expected effect on high-bypass ratio turbofan engines. If you want sound data on what is an acceptable level of ash and what it's long term effects are you will not get them from mathematical modelling, you will only get them from empirical evidence, and that means going out there and flying. The airframe and engine manufacturers can only gather so much information and experience from predictions and test flying. The rest does, and has to, come from airlines flying and reporting back their findings. It has always been the way, and it will always be the way, simply because an exhaustive program of testing by manufacturers whould be prohibitively lengthy.