I just did a simple calculation in my head.
The social cost for raising a child who is born in the United States up to the age of 25, when presumably they have left their home and got a job, is in the hundreds of thousands of dollars.
The cost of importing a child who has already demonstrated physical and mental health and who is younger than 25 is less than $10,000. A big difference.
Since the United States remains a magnet country for immigrants from the entire planet, it will always be possible for the United States to grow vigorously by importing desirable young foreigners.
I know there are some drawbacks to this observation. Some of the money these new Americans would earn would be sent back to their families. Some of these new immigrants would be supported by our welfare structure. Hopefully not many.
When I look at this option, which is politically possible, I can see that the American population has a long-term positive growth rate, regardless of domestic reproduction.