Immigration IS a threat. But the problem is that you're not specifying WHICH immigration.
Immigration of highly educated and intelligent people is a good thing in general. "Best and brightest", as they call it.
Immigration of criminals and uneducated farm works is not necessarily a good thing.
You can't be welcoming to the 'best and the brightest' if you treat foreigners in general as a threat, if you view their cultures and countries with disdain, if you treat all their unqualified friends and relatives as 'criminals' or 'idiots', and if you baldly wish to suck the 'best and the brightest' from other countries into your own.
Xenophobic attitudes like yours make the United States much less inviting for precisely the people you claim you need.