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.