Instead of a child, an adult must adapt to a very different culture. With lots of cultural clashes that are pretty obvious, especially in Europe, where a lot of the migrants come from the neighboring Islamic world with its very different social norms.

Not to mention that you rely on the fact that other nations do not stop reproducing. What if they do?

Countries like Bulgaria and Serbia have dropped into very low birthrates before becoming wealthy, thus they exist in the worst of worlds: educated and employable immigrants obviously choose places like England and Germany where they can make a lot more money, domestic youth migrates to the West too, leaving an old and vulnerable population without services.

Very true, but human beings can adapt. For example, all the Europeans that came to America 400-500 years ago are doing quite well today. The culture thing is just a red herring. All you are trying to do is maintain the status quo (nothing wrong with it).

The impact on finite, planetary resources is less taxed by immigration (1+0) than by birth (1+1).