I can assure you that assessments are quite real and seeable, when they have been done. Every ATS unit in Australia has a Group Operating Risk Assessment which covers items such as staff availability restrictions, loss of navaids, natural disasters affecting service delivery (eg cyclone wipes out the building where all the nice air traffic controllers work from), civil unrest etc etc.
That is where contingency plans are born, so that when an unusual or rare occurrence happens ATS providers are able to implement contingencies that restrict rather than curtail totally, operations. Obviously one of the contingencies may indeed be to close totally airspace, but not until after a proper and detailed assessment of the risk that is involved to aircraft.
If this process is not in place the real big question is what new evidence was revealed that allowed the airspace to reopen? A few test flights are not sufficient to go from total closure to fully open.
It beggars belief that such a mature aviation industry as is found in Europe does not seem to have such systems in place. To close the airspace over Frankfurt for example, without any hard evidence that the ash was anywhere near Germany let alone southern Germany, is very difficult to reconcile with the unilateral opening of the same airspace based on a couple of VFR jet flights.