This is due to recent immigration trends. Anglo-American culture has seen off much more substantial threats in the past. The people coming have even less economic or social power than, say, the Germans of the 19th century.
Realistically, if Spanish becomes the working class language of these regions it may come to dominate if the disappearance of the anglo middle class continues, but this is only in peripheral regions. The heart of US economic power is in places like New York, Boston, Seattle, San Francisco, which are not under any kind of linguistic pressure.