Other rich countries also have dogshit birthrates for native and every immigrants who comes in start getting similar birthrates one or two generations in. And rich countries usually have a lot of social benefits that can only be maintained by having more young than old people. You could try cutting off social security but this is gonna be really unpopular, especially with the old population who rely on it the most AND vote the most. Even if you completely remove that, having less people around just sucks in general for the economy and would led to a period of downturn that would spook most people. No country so far has figured out how to encourage people to fuck more, even though this is something that is happening across many different societies. So the easy short term solution to this demographic problem is to import new people, although it is far from being a perfect panacea.