You could make the case that population growth doesn't benefit 90% of Americans, it might goose GDP which is essential in a Keynesian system but it really isn't benefiting most people.
Especially when the growth is disproportionately in the form of people that are largely economically worthless.
Immigration should be more merit based at the very least. If you're the most prosperous country in the world, why would you just invite anyone in as a social policy?
I don't want government quotas, let the market decide what we need. Obviously there's a market here for Mexican workers, if there wasn't then they wouldn't be coming here? Also to the original question, if you got cancer, do you want the brightest of brightest fixing you, or are you okay with second, third or fourth best?