Various items I've read suggest immigration, even illegal, is a net plus for our economy. I realize they may well be false, I can't tell. I think more legal immigrants would help our SS situation. Bear in mind your SS benefits relate to how much you paid into the system over 35 years. If you live longer, you do get back more than you put in of course.
Japan has a MAJOR population crisis, they average 1.3 kids per couple, and the population is declining, and they basically don't allow immigration to any extent. It is one factor that is limiting their economy. My impression of Japan vicariously is that they are a declining country, not doing well at all.