>economies run on labor and skill, and growth or shrinking of a labor pool has serious ramifications. [...] In most countries with low birth rates, it would be extremely beneficial to everyone there to boost population growth through immigration.

It is possible to make the argument that negative birth rates are either (1) a result of overpopulation correcting itself or (2) a result of social problems relating to factors like marriage and career goals. I would consider (1) to be a strong form of this argument and (2) to be a weak form. (1) being true implies that there is essentially no way to maintain first world living standards at the current level of population in first world countries, and that the population actually should go down in the short term (say over the next few decades or so) until it reaches some balanced level at which the birth rate would likely naturally return to ~2.1 This seems like it may actually be the case in certain countries like Japan and Korea where most of the population lives in high density cities and it is even more clear in cities like Hong Kong which has a population density of 7,060 people per square kilometer[0] and a birth rate of 0.7.[1] and Singapore. If (1) is true, high immigration would always be a net negative in the long term. If (2) is true immigration is acting as a band-aid for solving that problem, and once that problem is solved the specific argument you gave in favor of immigration would not be valid anymore. I think both (1) and (2) are true to different degrees in different countries. It is hard to argue that the issue with eg. Singapore's low birth rate has nothing to do with it having the third highest population density in the world.[2-3] Some European cities also have extremely high density cities such as Paris, but there is considerable variability, which leads me to the conclusion that the answer involves both (1) and (2).

>It sucks when someone is outcompeted by immigrant labor, but that should probably prompt some self reflection -- how can someone new to a country substitute into their job? And why weren't they doing more to find a job where that wasn't possible?

One reason might be that immigrants are willing to work for less pay. Additionally immigrant families are more likely to live in denser housing, for example in multi-generational or multi-family households, to reduce cost of living,[4-6] which is something non-immigrants generally do not want to do. These are just two among perhaps dozens of factors. The two above factors are related to my earlier point about there potentially being "no way to maintain first world living standards at the current level of population in first world countries."

[0] https://data.worldbank.org/indicator/EN.POP.DNST?locations=H...

[1] https://data.worldbank.org/indicator/SP.DYN.TFRT.IN?location...

[2] https://data.worldbank.org/indicator/EN.POP.DNST?locations=S...

[3] https://data.worldbank.org/indicator/SP.DYN.TFRT.IN?location...

[4] https://www.cdph.ca.gov/Programs/OHE/Pages/HCI-Housing-Overc... - California. Data broken down by race rather than immigration status but should correlate with immigration status.

[5] https://data.census.gov/table/ACSST1Y2023.S0501 - US. See "1.01 or more occupants per room." Also compare average household size and average number of rooms.

[6] https://link.springer.com/article/10.1007/s12187-020-09743-7 - Finland