How many non-Einsteins from Africa and Asia would want to move to Western countries if given a chance? a few hundred million? A billion? How does a say Germany or France support a couple hundred more million people all at once?

I think open borders is inherently a call for decreasing standards of living for the populations of those countries where most of the migration would be directed to. I just can't picture a scenario where hundreds of millions of people move to my country within a short span of time and not completely destroy the infrastructure under the weight of sheer numbers.