Basically all developed countries already have birth rates below the replacement rate. In the US each women on average gives birth to 1.64 children, in Germany there it's 1.5 per women, in Japan 1.33 and in Korea 0.84. To keep population constant without immigration, the average women would have to give birth to slightly above 2 children.

The only reason population numbers aren't collapsing is immigration, but that comes with its own issues. Extending live expectency and productive lifetime isn't that crazy (if we can get a handle on our rising living standards and resource consumption).

https://data.oecd.org/pop/fertility-rates.htm