Honestly, the need to adapt isn’t because of wanting to succeed or wanting to be mainstream. Children who are born in the US, just don’t have cultural ties to anywhere else.
When I moved to Georgia, (age 7) I lived in an English speaking area, with little hispanic influence. I learned to speak English because I wanted to, more than that I was forced to. I entered the English speaking school, and wanted to be there. Had I not been able to keep up, my parents were prepared to move me to a Spanish school nearby. I wanted to stay.
My children live in a very hispanic area, where there are plenty of Spanish speaking stores, people and schools. They can attend Spanish Mass. They are all able to speak fluently, but they reject the whole thing. They refuse to speak in Spanish, even when I speak it first. If you asked them why, they would tell you that it sounds “stupid” or something equally as meaningless. What they really mean, is that they want to be different from their parents and grandparents. They are American, and they speak English. They have no cultural ties to Brazil or to Puerto Rico.
It is a natural progression, and has happened to everyone I know in a similar manner. Cultural ties are strong and they take a while to break. The first generation born and raised in the US, is always more American than the generation that immigrated. It is caused by a need for independence more than the necessity of survival. I seriously doubt that the availability of Univision or other Spanish language programs have any effect on whether immigrants learn English or not. If they want to, they will. If not they won’t.
Your experience may be typical of the East coast. Brazil and P.R. are far away across the ocean. In L.A., there are constant cultural ties and the border is three hours South. Families are constantly going back and forth across the border, both visiting and financially supporting the folks "back home". Most of the ones I meet are not interested in becoming "Americans". They want to be "Mexicanos", earn what they can and game the system for the rest.
I'm not speaking out of my hat here either. I lived for 9 years in the East LA barrio of Lincoln Heights, about 8 block from "La Plaza de la Raza". Literally everyone else living on our block were 1st and 2nd generation Mexican or Chinese immigrant. A few of the 2nd generation neighbors spoke broken English at best, having never left the barrio except when visiting Mexico. The majority of rest were bilingual, but usually made a game of "no habla" to mess with the "gringos". A very few, such as our landlord, were fluent and used English first. They were the only ones who were making anything of themselves.
Except for the elderly man who lived downstairs, I don't think I ever saw a first generation Mexican speaking English to their children in public. That gentleman was old school, as you describe; a proud naturalized citizen, who spoke bitterly of his son, who had returned to Mexico, and would drill hid grandkids in English when they would visit.