volcanic ash has happened in different parts of the world since aviation begun
Since the beginning of aviation engines have developed quite a bit. Pressure ratio and hence peak temperature have increased significantly since the De Havilland Ghost (actually the other way round, higher temperature capability of modern material has allowed the pressure ratio to increase, but temperature is the critical parameter when discussing melting ash). Modern engines depend on electronics and sensors, readings out of the normal due to clogged pressure sensors or contaminated temperature probes will trigger a shutdown or run the engine out of its intended parameters which will not improve its performance and life. Modern engines have highly sophisticated internal air cooling of some turbine blades through tiny holes, which may clogg. Modern engines internal aerodynamics are cutting edge technology these days, they do not take that much of molten lava in the turbine as older models.
But nobody knows the exact limits. There are no requirements, there are no standards. There is not even standard ash available, every vulcano is different.
So it is fine to be a bit conservative, but 2010 was far over the top. The 2014 statistics is already poor, we do not need another airliner in the dust (or the ocean).