Immigrants get older like the rest of us, no kidding. They are subject to the same economic pressure that prevent starting families like the rest of us. Their kids grow up with the same social influences and become lazy/entitled at the same rate our's do.
So the immigration growth agenda requires a steady heavy stream of new arrivals replacing the ones who become "corrupted" after a few years in this country.
It is clearly unsustainable and its advocates either know that and are too greedy to care, or they are being dishonest about their intentions. Probably a little of both. The answer lays elsewhere.