The Mexicanization of America is very recent:

The vast majority of the ~40 million Mexicans living in the United States today are post-1970s immigrants and their offspring.

In 1850, the freshly annexed southwestern states were home to 83,727 Mexicans in total. Arizona and Colorado were 0% Mexican, Texas 5%, California 16%, and New Mexico 98%.

In 1970, ~5 million Mexicans lived in the United States, of whom ~760,000 were first-generation immigrants.