Given this scenario, why would humanity devolve? No knowledge is lost. Most everyone is much poorer, but that's not a big deal on the "back to stone age" level. We can still build machinery and computers, we can generate electricity, etc. etc.

Agreed, we could probably recover from a natural disaster, or even a war. On the other hand improperly handling the current migrant crisis in Europe may very well ultimately be as disastrous as Emperor Valens' decision to let the Visigothic refugees fleeing the Huns settle south of the Danube.

Ah, got it. I'd probably agree that if 0.7 billion people just randomly dropped dead, the long term effects would likely be completely recoverable. It would be one more massive tragedy in the history of the human race, but not our downfall.

However, people fight awfully hard to survive, so in real life such massive death tolls often happen only after a long, brutal fight for survival. So, GCRs may be accompanied by massive cultural, political, and economic changes. The dust bowl killed very few people, but displaced millions. If 10% of the world died of hunger, I would expect this to coincide with a larger, more severe form of the great depression. However, if the super-volcano or whatever just killed 10% instantly, without the dust in the atmosphere blotting out the sun and inducing years of winter, then I agree that it's not an existential risk.

Note for posterity: Apparently the depression started before the dust bowl, although it seems to have been made significantly worse by it. I don't think this negates my conclusion, although it made me update in your direction somewhat. I can track down a couple other potential examples to examine, if you are interested.

The conclusion that Xrisk doesn't necessarily scale with number of deaths is potentially useful. It hints that there may be efficient ways to reduce X-risk, even in the face of massive death tolls. In cases where deaths are unavoidable, perhaps there are effective ways to contain the suffering to only one generation.