Check back in a few years, right now its good, and we wont see results for at least a few years:

https://www.populationpyramid.net/united-states-of-america/2...

>how do you justify labelling a birth rate that's been below replacement for 18 years as "OK"?

The US being a nation of immigrants has this unique selling point of attracting people to make up for their woes such as birth rate. Unless we see millions of citizens getting deported, the people "voluntarily" relocating don't move the needle in this metric.