That is not the problem. Population declines as countries move to first world status, and I think the people not having kids due to financial constraints are few and far between. Otherwise population would not start and continue diminishing as an area becomes more affluent. People have less (or no) kids because they don't want kids, don't want a bunch of kids, and can reasonably expect the kids they have to survive to adulthood. And access to birth control, education, and other opportunities (mostly for women) makes having less kids (by their own desire) possible.

So bringing capable workers in means they pay into taxes that support the aging and school-age population, and never had to have their school-age years paid for. They're a productive member with half the cost over their lifetime.

It's a no-brainer... as long as you're not worried about changing the... shade or hue... of your population over time.