Minor nitpick - Sam is not the one making these points in the referenced episode - it's the guest who wrote a book on the topic. In fact Sam brought another guy onto this show to act as a backstop, admitting up front that he didn't feel he knew enough about the topic to have a deep conversation without some help. The other guy pushed back on various points that Zeihan made and it's a pretty interesting discussion.

As for immigration, no arguments there. But that point leads back to the question of why a society hits population decline in the first place. Presumably the same things that cause your "native" population to stop reproducing eventually affects your immigrants after a generation or so, so while it can definitely slow the population decline, it's not a full solution.

To your point about the loss of deep expertise - I think the problem is still worth thinking about. Yes, we can make new experts. And those experts might even be more productive than the last generation of experts. But at the end of the day, it takes longer to make them, and you still have less.

I'm not sure how much I agree with Zeihan, but he makes some interesting points which I hadn't previously considered.