I hear zoos have trouble getting pandas to mate. Why don't they just add some polar bears, or leopards, or even rabbits?
What do you think the pandas would say if they could talk?
A state is a three-way bargain between the living, the state, and the unborn. The state imposes all sorts of inconveniences on us---why do we put up with it?
The constitution lists several reasons---common defense, general welfare, and to "secure the Blessings of Liberty to ourselves and our Posterity"
Imagine, privileging one's own children over someone else's! How aggressively family-ist.
In other words, one of the primary purposes of a state is to act as a vehicle for inheritance.
That's why immigration is not a solution---the point of a state isn't "people," it's our children, specifically. Without that, no one has skin in the game.