The birth rates are low because of the terrible environment that doesn’t support having and raising children. All you’re doing is importing more people who will also barely have any children within a generation or so. Mass immigration is just throwing bodies at the bottom of the pyramid scheme. You can see this in action in Canada where housing is absolutely unaffordable, but large numbers of immigrants are brought in who have to work for shitty wages and live with multiple families in a single rental unit.
The screaming about low birth rate is because corporations want to keep a high labor pool so they can drive down the price of labor while keeping up demand for consumption.