The low birth rates are a concern because they need the bodies to generate revenue so they can take care of old people who will lean on government services.

Why should you care about population decline? Fewer people are good for the climate, but the economic consequences are severe. In the 1960s, there were six people of working age for every retired person. Today, the ratio is three-to-one. By 2035, it will be two-to-one.

Some say we must learn to curb our obsession with growth, to become less consumer-obsessed, to learn to manage with a smaller population. That sounds very attractive. But who will buy the stuff you sell? Who will pay for your healthcare and pension when you get old?

https://www.weforum.org/agenda/2021/06/birthrates-declining-globally-why-matters/

Ok, so what does that have to do with your question…

What age group tends to be the biggest voting bloc, the most xenophobic, the most nationalistic…? Old people. Get those foreign people off my lawn, keep those foreign drug dealers, thieves, and layabouts out of my country.

Then there’s exploitation of foreign labor to undercut wages and work rules of citizens of the country the labor is being imported into.

I’m sure there’s more, but basically it’s a hefty dose of xeonphobia and nationalism along with groups not wanting to literally lose jobs to someone who will do it for a lot less.