It’s a problem that there will be fewer people in the generation below ours to support our generation in our dotage. This problem is the same regardless of your economic model. Fewer people in the working pool and more people sick and elderly is a bad time.
Population collapse is also occurring in communist countries. China additionally scuppered themselves by previously putting taxes on multi-child families to get ahead of overpopulation and in doing so accelerated the collapse of the working population ahead of that in capitalist countries.