Overreaction? don't think so.
Flying through volcanic ash is not a good idea. Depending of course on the concentration!
European aviation is regulated by an amalgamation of polyglot officials, penetrated, tested and stretched by national and private enterprises of widely differing standards. Under the circumstances, the regulators have done a reasonable job under pressure from despairing airlines yearning to leap into the air once more!
As quoted in the Wall Street Journal, Ken Williams, fleet captain of Alaska Airlines, says that they treat ash clouds with the greatest respect. When Captain Williams receives an alert, aircraft of various types, without passengers, are flown to measure winds and temperatures, providing additional data to be able to map and avoid ash clouds. Alaska pilots are given plenty of simulator practice in dealing with ash.
The Alaska Volcano Observatory and the United States Geological Survey make it their business to observe, classify and notify Volcano ash hazards to aviation. Says the USGS, "Volcanic activity threatens safe air travel when finely pulverized glassy abrasive volcanic materiel is explosively erupted into the atmosphere and dispersed ... in flight paths. ... Ash can cause jet engines to fail in flight..."
Iceland, as a nation upstream from Europe, frequently suffers from volcanic activity. Now that winds have shifted, its their turn to shut down the airports. Clearly, Europe as a whole will need to sponsor the equivalent of the Alaska Volcano Observatory and the United States Geological Survey as a matter of urgency. Sending test beds full of pax on ETOPs anywhere near ash clouds is not the ideal soluntion.