[anbu32]

"because the population growth rates or even the populations are already declining."

The population of both the us and w.europe is growing. (Immigration)

[danbruc]

But migration and the normal population growth are qualitatively different - normal population growth is an exponential phenomenon given a constant growth rate, migration is a linear phenomenon given a constant migration rate. And at least in developed countries I see now real issues controlling both forms of growth, limiting the number of immigrants as well as establishing a two children policy if required.