Slow population loss, while concerning for policy makers, can be managed theoretically by moving money around. Taxation, subsidies, etc.
The US is currently at 1.6 fertility rate. 2.1 is replacement rate, so a pretty steep drop of 25% loss per generation. But we have substantial immigration to make up the shortfall. It’s an issue, and it’s trending down, but manageable for now.
Fertility rates of 1 or less are terrifying. Each generation is half the size of the one before. Half as many workers supporting the elderly. Retirement/pension systems will be strained then collapse, allowing retirees to fall into poverty. Half as many workers to maintain infrastructure, half as many doctors, half as many nurses, half as many experts in every field, means half as many researchers making discoveries and breakthroughs.
God forbid you go to war and have half as many soldiers to call on, from a workforce already stretched beyond any before. It’s a recipe for mass suffering in a scale never before seen.
South Korea and Japan are currently below 1. China might be even lower. People are, generally, resilient and resourceful. Adjustments will be made. People will work into their 70’s and 80’s because there is work to be done. But there will be a great deal of suffering.