I am bilingual, but I'm here to tell you that when I do want or need to speak Spanish, it's HARD. Takes a little extra brain power because my mind is wired in English, which is my first language.
When I've gone to Mexico on vacation, I resort to English when I'm just too lazy to speak Spanish or had a few too many margaritas.
So, based on my own experience, I think those who speak Spanish here and refuse to speak/learn English and use it as their first language are like me in Mexico at times--lazy.
I know what you mean...when I travel I always speak English unless I have to speak Spanish (or Russian). That's why I think that First Wave Immigrants assimilate much less quickly than their children do---the children have brains hard-wired (I like your term) in English and Spanish, while the parents grew up speaking only Spanish.
Sounds reasonable..my hubby uses spanish alot in the ER