Well, it is not at all that naive situation, so I think it is good move to close airspace for all flights.
I beleive there was an accident involving BA flight somewhere close to Java some time ago, when aircraft flew through a cloud of Vulcanic dust, all engines failed, but fortunately crew menaged to restart the engines on safe enough distance to rescue the airplane.
I have heart on the news that several NATO planes (I belive F-16) have tried to flew trough the cloud, all menaged to land but dust particles similar to glass were found inside the engine...