German used to be everywhere, being heard in the streets, with large numbers of German-language newspapers and cultural clubs. The Germans, being Germans, liked to organize themselves into clubs.

It gradually died out, as immigrants stopped teaching it to their children. Usually the children of immigrants learned the language at home, but they didn't usually pass it on to their children. World Wars I and II did a huge amount to suppress German culture and language, but it would have eventually died out anyways.

By World War II, German was still common enough that the Army was able to find large numbers of German interpreters among the young men who had spoken it with their parents.

My grandmother knew German as a child, but the language in the family had switched to English by the time she was about 10 years old. Once she was grown, she wasn't married to a German speaker, and hadn't spoken it in years anyway, so none of her descendants learned it. By the time I talked to her about it in her 80s, the only German she could remember were a few lines of some children's poem she recited when she was a child.

I suspect that this will eventually happen to Spanish, although the continued migration of Spanish-speaking immigrants will slow down the process. Eventually, the non-Spanish-speaking descendants will greatly outnumber those who speak Spanish, and the language will slowly decline.

I wouldn't be so sure for that one, the thing is unlike German, there is a country with 120M of Spanish Speaking people a few miles away, as Mexico's GDP keeps growing, US economy will become even more dependent of Mexico's trade. My point is people will get more proficient at Spanish over time, not less.