That's a very good point, actually. Sometimes I forget Calamities take a lot of work and planning to cause. They can take centuries or even millennia to plan. It isn't just destruction, but a specific kind of destruction. It has to be an aspect of aether, such as light, dark, earth, water, ice, fire, etc. And then there has to be the corresponding catastrophe on a Shard that matches the imbalanced aether on the Source.

Koryu certainly has the destruction part down pat. I'm guessing his aspect would be fire? But the Ascians would have to make sure that same aspect was overflowing from a Shard. Still, though, I'm sure they could have figured something out. Unless Koryu was too unpredictable, even for the Ascians.