>although consumption is much higher there is still such a need for continued population growth in western nations that immigration is used to ensure there are enough younger workers to support the wealthier aging populations

It's because social-security/pension systems are a pyramid schemes that can't sustain themselves without ever growing population. I consider many Western EU countries overpopulated (UK, Italy, etc), Google Maps satellite images also do a pretty good job at portraying at just how overpopulated the earth is. Pop growth every decade is still insane.