Not only does it decline, but it declines exponentially. This is why fertility changes will have a far bigger impact on societies than most people appreciate. If a country has a fertility rate of 1 (and places like South Korea are now well below that even) then that means each successive generation is 50% smaller. And this change is compounding. So after 5 generations, for instance, your generational size will be 97% smaller (1 - 0.5^5).

And all the while this is happening your economy is getting wrecked not only because suddenly you have far fewer people, but because your society turns into a giant retirement home, as fertility also determines the age ratios within a society. You can see this by working backwards. Again taking a fertility rate of 1 as an example, if we have 1 newborn then we have 2 20-40 year olds, 4 40-60 year olds, 8 60-80 year olds, and so on. So you have a shrinking economy paired alongside an even smaller ratio of workers, and much greater social strains on that economy as well.

This is also why immigration isn't anything more than a very short-term bandaid. When you're speaking of population declines near 100% over such small windows of time, you end up needing to basically replace the entire population of a region. Doing this with immigration, let alone as this is increasingly a global crisis, just isn't realistic. And this is completely ignoring the social consequences of such actions.