It’s all about a) the amount (enough to make up for the lower developed birthdate, and to keep inflation low, but not so much as to completely destabilize the quality of life of the native lower/middle class) b) composition (not all immigrants are created equal when it comes to net contribution (regardless of desire to work/contribute)(e.g. a someone older, with serious health conditions and lacking an a grade school education Vs say a college educated, young, and healthy.)