As always, commercial pressures have a way of separating the men from the boys. This is particularly so in cases which lack a measurable, repeatable outcome. That said, I should qualify my criticism by stating that I have absolutely no knowledge of what re-routing possibilities are being discounted.
There is wide room for additional testing and research into the volcanic ash phenomena. But I rather doubt that anyone is going to obtain enough repeatable data with which to derive a metric allowing a go/no go decision...not today, and not tomorrow. One or two or a dozen successful test flights say nothing, because the degradations leading to an accident do not function on a linear scale. To my knowledge, there are no tools to measure where the cloud is hazardous and where it is not in real time; perhaps someone in the research community has had an epiphany in the past couple of days, but I doubt it. At present, the cloud is either there or it isn't.
Any decision to operate based on the idea that the ash is there but not harmful is nothing less than ignorant and irresponsible, regardless of its subsequent success. Indeed, it is likely to ultimately lead to a much more cavalier operational attitude toward this problem in the future...until some unlucky sap can't get them restarted again.