It's a better measure than GDP/S&P/401(k) line-go-up especially [re: America] when the native Euro-based population has been aging and dropping for decades, once you strip away all the post Hart-Cellar immigrant lineages.
Let’s play a thought experiment.
Let’s say we have a million people that are so technically sophisticated that they are a space faring civilization capable of seeding the universe with living ecosystems capable of perpetuating life and evolutionary processes. But they are entirely infertile and will never give birth to another individual of their species.
And we have another population that doubles every single year but is incapable of leaving their home planet.
Which one is more valuable?
It depends on what your measure of value is, but if it is to maximize the amount of life in the universe, then population growth is not the right metric, expansion of life through technological means is the more appropriate metric.