Just like population growth will not continue indefinitely, neither will population decline. Below-replacement levels signal that there are a lot of individual

Just like population growth will not continue indefinitely, neither will population decline. Below-replacement levels signal that there are a lot of individuals in the pool who are not pre-disposed to reproduce in their current environment. Over time, this will be selected against, those individuals who are pre-disposed to reproduce will produce more individuals similar to them. That pre-disposition is genetic to some degree, but it is also cultural. Our society tells women that it's okay to defer birth or to not have children at all. Other societies pressure women to be married with children by the age of 25, before their fertility declines. From a moral perspective, the former may be preferable, but from a population maintenance perspective, obviously the latter is advantageous. As for social welfare, obviously there's a looming crisis of elderly care, but it can be mitigated to some degree by immigration.