The biggest plus for a continuing German presence is that if German instead of English is the first language of science, engineering, and medicine many Americans who have no family connection to German will learn it at the secondary or university level. Because of the Spanish speaking proximity of South America Spanish will still be a useful language for many Americans, but German will be widely taught (still as it was prior to WWI) and probably the major second language of the USA. IMHO while by 3rd generation German speakers will probably have to learn their German in school, but you'll still see German newspapers, bilingual German/English schools, churches with services in German. Unlike many languages, like Polish, there will be a benefit to keeping ability in the "old" language, due to its international use.