Birthrate is irrelevant.
It is just an excuse.
Invasion kills.
Birthrate is highly relevant to the survival of any civilization. A sustained below-replacement birthrate will collapse any civilization within a few generations. Gradually and then suddenly, the demographic math turns very cruel.
The decline is even more dramatic for welfare states whose missing taxpayers were never born; their Ponzi schemes are facing collapse for lack of new paying participants. The "solution" is an invasion of nonassimilating hordes from the Third World.