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.

Tldr:

Having too many cultures that have not established a "market share" in politics makes the, people who run a country, job harder as it has to contend with dealing with the potential of new cultures forming and the inevitable culture clashes that follow as differing values and ideals will demand different things.

It fractures and dilutes points of control which encourages politics to try ensure loyalty though aligning itself with views of the majority.

End tldr

Unironically, Stellaris is probably a decent example of the thought experiment played out. Unless a species is built with ideals of the intergration and/or has its proper foundation set then it can quickly spiral out of hand as you have to deal with " a hunded voices asking for one thing".

It is far easier to control and secure a foundational majority based off of one species as it can be more easily guided towards an established outcome.

Adding too many "outsiders" has the potential to cause an imbalance and a shift in thinking which then requires a new paradigm to "herd the sheep" as it were, while still trying to maintain a standard that the base species has become accustomed to.

If it not carefully controlled, it can potentially lead to a fracturing of opinion and thoughts which is a lot harder to manage and "guide" as one runs the risk of isolating one group and in doing so opening up the potential cascade of problems if the ignored minorities builds up steam which then forces leaders to contend with trying to figure out a way to maintain control over the many species bases while still doing it in a way that causes the least amount of disruption to their control.

Because racism

This is a part of the SNP's manifesto already, FYI.

God, the comments in this thread are nearly all awful