But most people moving in don't cut all ties with their home country nor never touch their passport again I think.
Again, a huge percentage of the population has divorced parents, plus all the kids born out of wedlock. That is like half the population that already don’t have the same last name as at least one of their parents. Everyone who deals with the population is going to encounter this situation every day. They aren’t going to be surprised or confused by a kid with a different name as their parent.