That's a very superficial analysis. These engineers are not native to Mountain View, usually not even native to California, and sometimes not native to the US.

That's a very superficial analysis. These engineers are not native to Mountain View, usually not even native to California, and sometimes not native to the US. Several of the highly productive engineers born and raised in Paris moved to Mountain View, because their in-demand skills allowed them to get a high paying job. Their worth is not set by the Mountain View office - they were moved there because of their worth. If you remove those borders, you still have the same highly skilled engineers in demand, the only difference is that a company doesn't need to pay for their visa and relocation anymore.