I grew up in Southern California in the 1970s, and a non-trival percentage of the daily conversations I had with normal people had to be in Spanish, because their English was much worse than my Spanish, which was fair.

"Southern California" was/is a very, very big area. I grew up southeast of downtown.

Can you share where/when you grew up? Just curious.

As far as 'replacement': I can't dismiss such opinions out of hand, but for another anecdote: two of my great grandparents, who came from Germany in the late 19th century, lived in the US for many decades and never learned English that well.

Their children, my dad's father, of course spoke English fluently.

I guess I’m not sure I understand what’s so great about English anyway. Why not switch to Spanish, or at least adopt a more bilingual approach where it makes sense.