Just like they were 200 years ago!
The more you try to prohibit something, the more you (a) make it sexy and cool, and (b) spend of your resources trying to enforce the prohibition.
I'm not defending illegal immigration. Neither are a lot of Mexican-Americans (most of whom are loyal, law-abiding, hard-working citizens).
What we need to do to protect and promote the POSITIVE parts of our culture. Maintain a strong border, but don't spend a lot of time and energy fighting a Hispanic culture that has been there even longer than our own.
We most certainly did not have predominantly Mexican areas 200 years ago. A few settlements but whole cities and cultures? You'll have to source that. That means they all left then started coming back in the last 30 years or so after we built the areas up.
You are totally defending illegal immigration by your comment that they were here first and had their culture here first.
I don't know where you live but here a lot of them don't want out culture no matter how we promote it. They want their culture and what they consider their land back and don't respect our illegitimate control of their land.
They come up here in droves without any concern for our laws other than not getting caught.