Population decline. Which in itself may not be a problem but has all sorts of interesting economical consequences.

You get an aging population requiring care, with less people to both physically and financially/economically fund them.

You also move from always having to build new houses and infrastructure to suddenly having excess housing and infrastructure that isn't needed. Not only does that eliminate a whole bunch of jobs and related economic activity but also drives prices down, etc.

I am sure this is an absolutely terrible summary but you can google population bomb, population decline, etc, for much more informed takes. But basically for the last 100 years the entire world and economy has run on an ever-inflating path and that could change at both a country and world level.

After a period of decline, it'll eventually fall off a cliff and weird economic after effects happen.

Try reading https://longnow.org/ideas/the-heresy-of-decline/