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.
While thinking about my own next career steps, I've been writing down some of my thoughts about what's in an impactful career.
In the process, I wrote an introductory report on what seem to me to be practical approaches to problems in catastrophic risks. It's intended to complement the analysis that 80,000 Hours provides by thinking about what general roles we ought to perform, rather than analysing specific careers and jobs, and by focusing specifically on existential risks.
I'm happy to receive feedback on it, positive and negative.
Here it is: Reducing Catastrophic Risks, A Practical Introduction.