You only need to gain immigrants to do work that natives won't. You don't need those immigrants to breed, no more than you need the natives to keep breeding. A

You only need to gain immigrants to do work that natives won't. You don't need those immigrants to breed, no more than you need the natives to keep breeding. As more and more jobs become obsolete, a country will be able to run just fine on fewer people, so long as there are enough people to do the work that can't be automated.