Anti-immigration..does Right wing still fools masses?

[Angrboda]

Stop the decline of which population and why is that something to worry about?

Alot of reasons, but, in a nutshell, sols - and at some point a population is not large enough to support itself or it's standard of living.  This is why demographic decline concerns some people.  

That's the bargain that countries make with immigration.  We're stealing labor out from the other guy to cover whatever "native" birth rates can't match.

Said population isn't an isolated system.