Your thread title talks population but the content of your post and most subsequent post talks about immigration.

Purely population wise, yes, economics likes more population, especially in the US as so much of it is dependent upon consumerism more people alive = more people buying things and making things. The US faces economic disaster much like Japan's if US birth rate continues to fall. Something that only immigration could really stem. As many 2nd generation plus women are not interested in having 3 plus babies.

Global health wise (both the planet and humans) could certainly do with billions less humans. Capitalism may or may not be the best system of economics devised so far, but we should, and need to, come up with better. Perhaps capitalism can continue to thrive in a mostly virtual world, where endless growth is only limited by how efficient processors are and perhaps even with very efficient processing, how much electricity we can muster in an efficient manner. Something that can be greatly helped along by giving everyone 10 gigabit internet access allowing all processing to be done on highly efficient processing farms.

In the real world with rising populations and dwindling resources pure capitalism can not deal with an end to growth scenario and capitalism highly encourages constant growth. Imagine Japan's economic stagnation, but much worse, and on a global scale, so there is no escape.