That's fine, but now you need a way to cope with an outsized elderly population. Not just funding their benefits, but also providing them care, etc. Without population growth, this becomes an extremely expensive situation.

Because of global urbanization, that's a problem the whole world is going to have soon enough. Japan is leading the way and the US is slowly following. But, the trend is that in a hundred years the entire planet will be having sub-replacement birthrate. Yay. The population explosion crisis has accidentally been averted. Oops. Now we have the opposite crisis to deal with instead. Might as well start figuring it out early.