One of the key points he made that seemed fairly intuitive is that as your population declines you eventually wind up with significantly fewer long term experts in various fields of science and engineering. This means that the people who train up the next generation of scientists and engineers are spread thinner, and therefore less depth of expertise in those fields as the decline continues.
I'm not sure how much that idea is backed up by evidence, but it at least makes some sense.
Population decline had never really registered as a problem to me until I heard that episode - I always kinda figured that less people around would be a net good. Definitely worth a listen if you're interested in the topic.