The problem is that the migrants crossing into Italy don't speak Italian (or English), tend to have very minimal education or skills, so the idea that you can integrate them into society as skilled and productive workers is fantasy.
As I said, the subject is considerably more complicated than a 4-page summary, and it really comes down to what type of migration you have, who wins and loses, and other constraints on the society in question.
Maybe reverse the decline in birth rates, or get used to having a smaller population and economy, the economy isn't going to cease to exist simply because the population falls from 70 million to 60 million.