My father’s parents came from Germany and learned to speak English fluently. One major incentive that’s not mentioned here is that during WWI, it was ILLEGAL TO SPEAK GERMAN IN PUBLIC in many places. My great-grandfather was essentially under house arrest because his English was poor.
So, you support the suspension of Civil Liberties (as happenned to German Americans during WWI) in order to get people to speak a language? Sieg Heil! ;-)