

This is the way it used to be. Cities didn’t used to be able to sustain themselves through the birthrate alone; they were so toxic and dangerous that they would eat the populations within them, and needed a continuous flow of people from the countryside to sustain the population. Who would then, as the years went by, get fed into the maw and replaced in their turn.
Doing the whole country that way hasn’t been tried before to my knowledge, but what the hell, we might as well be the first to give it a shot.
Hm… maybe I am wrong. It’s definitely not just a conservative talking point, it was how historians looked at early modern cities for a while, I thought. But it seems like modern historians aren’t sure that’s the case:
https://www.jstor.org/stable/2598176
With just a quick look around, I couldn’t find anything that seemed definitive in the other direction, but also, the little preview that shows of that paper seems like it does a pretty good job of saying “Yo the reasons they said this is true are incredibly weak when you dig into them.” So maybe it was just premodern science from the leeches-and-ECT days.
There’s also this. Deaths are exceeding births in almost half the US, now:
https://carsey.unh.edu/publication/deaths-exceeded-births-nearly-half-us-counties-last-year
(And, of course, it’s mostly in the rural areas)