The OP is on target. Once upon a time, in a shtetl far far away Yidish was the common Jewish language, the mamma loshon of European Jewry. For better of for worse most Yidden today neither speak it or understand it. It is true that in certain communities it is strongly propped up and succeeds in becoming a 2nd language to English in the US and to Ivrit in Israel.
Be that as it may, many of the Gedolim heard shiurim from their Rebbeim in Yiddish and spoke it in their homes it is therefore a language very much beloved to them. And what is beloved to the Gedolim is beloved to Klal Israel. I, for one, regret never having learned Yiddish. Many shiurim are available on Kol Lashon from some of our greatest luminaries, in Yiddish.