It is blatantly obvious and nearly indisputable that the absolute best thing that could be done for the poor of the world is to allow them to immigrate, with or without eligibility for welfare services, to rich countries.
1. It would make the poor better off by obscene amounts. 20-100x earning potential.
2. It has, according to most meta-studies, very limited impact on natives.
3. Therefore, there is NOTHING known to humankind that would more improve average quality of life for humans on earth than allowing A LOT more immigration from poor to rich countries.
1. It might hurt some of US now. THEY don't count.
2. It might screw with the culture that causes the good that we have.
3. THEY might get welfare, or their kids might, which screws us all.
As usual for ALL arguments over all topics, there's very only minor disputes about the statements above. The real dispute is over what's important. And on that, rational folks disagree. Rather aggressively.
Whether you're pro- or anti- immigration comes down to which of these arguments you care more about. I care more about the pro- side's issues, as I am near 100% anti-tribalist (I don't have that gene, nor do I have a tribe)...and consider tribalists of all shades moderately bad people. Therefore I am pro-immigration. Ditto Caplan, etc. Others who are pro-tribalism, will obviously have different concerns, and therefore different conclusions.