read tragedy & hope. the short version is that due to how interest-bearing debt works with the economy, if the population stops growing then interest payments suck up excess wealth generation which causes recession which causes banks to collapse which implodes the capitalist system which then enters a death spiral. there are ways to ease out of this, and even ways to just endure it like japan has been doing for decades, but it's not fun for the elites at the top who have myopia and can only see one quarter ahead, so they tried importing migrants to fill the population gap and it doesn't really work and now they're out of ideas.
United States