Good question. It seems to be a truism that we should want to see the population growing.
Is it because many places in America where population has declined also tend to have social/economic problems, that people believe the population should always be growing? I don’t know the answer but that’s the sort of talk I’ve heard.
If the population collapses, the ponzi schemes fail, and the GDP doesn’t keep pace with the currency inflation, so the economy collapses.