You're completely correct to highlight this, it's a coming challenge to be sure. Developed nations need to fine a way to increase indigenous fertility rates without simply replacing their own populations and culture with immigration. For my part I think the solution is multi-pronged:

- Make child rearing more financially feasible (big tax cuts for families)

- Foster reliance on family and local social networks, rather than reliance on the state

- Nudge younger generations to take responsibility for the future of their nation, rather that the nihilistic individualism that currently pervades our culture

- Etc