>cannot maintain its economy - at least in the expected way, that is of ever-increasing GDP as has been the trend for the last 75+ years - based upon birth rate alone. We are below the replacement rate.

For analysis of any arbitrary economy, why should the economic-performance layer be in any way coupled to the number-of-people-living-in-the-country layer? I've been through two microeconomics classes, and one macroeconomics class, and none of the professors said "By the way in order for our theories to hold up you need to birth more people, so get cracking."