I'm not going to accept your premise that we don't have a unique culture worth preserving. You know, the one that is 'utterly contaminated and adulterated'.
A Nation is defined by geographic borders, and the culture conained within the people who reside there. A common language, or at least an agreed upon standard, is also indespensable. The laws that a people decide upon is the final mortar which preserves such a defined idea beyond the emotional instabilities of human nature.
I get this list from Biblical wisdom. Particularly the era of Babel, and the founding of Israel through geographic conquest and the establishment of the law.
There is a legitimate role for government to play, and the enforcement of borders is certainly one of them. Now, being that we are a government of, by and for the people, I happen to take particular offense to the idea that citizens who do the jobs that the government agents don't want to do are subversive vigilantes.
If anything defines citizenship, it's the role regular people have played to force this debate to to the front.
But your post dovetails my point in other threads that the underlying cause of this problem is the abhorrent corruption and racism in Mexico. If that were changed, this migration wouldn't be necessary.