>>12229383>>122294321) modern western societies already have a fairly low birthrate among non-immigrants
2) conversely, new immigrants have a high birth rate
3) immigration reduces resource strain on the countries they're immigrating from, allowing for those countries to continue to reproduce
Immigration, through demographic change, effectively brings the birth rate of poorer countries to richer countries and couples them with the resources of richer countries; yes, later-generation immigrants begin to match the birthrates of the native population, but that doesn't matter because there's always more immigrants.
Likewise, immigration doesn't alleviate the socio-economic issues of the poor countries they come from; all it does is produce an endless font of new population to consume more resources in rich countries.