You're talking mostly about slowing growth or difficulties in rebuilding. But my question is different: given the death of ~10% of the population, why would the civilization collapse at all?

Let's make it a bit more concrete. Assume the Yellowstone supervolcano unexpectedly blows up. Much of North America is rendered temporarily uninhabitable, there are a few years without summer leading to crop failures all around the world with consequent famines. Let's say 10-20% of the people on Earth die within, say, three years.

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.