> None of this people opining “just learn the language” have learned a second language while working as adults, let alone learned German.

I am very certain this is not true. The "just learn the language" people are typically rather people who are very talented in learning new languages (and often indeed to this as adults as a personal hobby - often even with languages from very different families), and thus are often not easy to convince that not everybody is as talented in language learning as they are.

Believe me, I know this kind of people:

I just want to quote some polyglot person who very casually said: "Being fluent in five languages is not something to be proud of - this is rather minimum standard." (she had the opinion that rather keeping fluent in 10 [!] languages is something that takes steady learning efforts to retain the obtained level in all of the 10 languages).

I've always heard it said that "polyglot" starts with the fourth language. The opinion is that to have a at-home (family) language, an outdoor (regional) language and a market (capital) language is too common to be considered truly exceptional. Once you pick up the fourth, you're getting into hobby territory.

Kató Lomb (maybe the first official "simultaneous translator" wrote about gaining, losing, and maintaining languages entertainingly and in detail.

edit:

"Polyglot: How I Learn Languages" https://www.tesl-ej.org/books/lomb-2nd-Ed.pdf

"With Languages in Mind: Musings of a Polyglot" https://www.tesl-ej.org/pdf/ej78/WLIM.pdf