Rashi – He probably spoke a Jewish dialect of Old French (written in Hebrew letters). That community was destroyed by the Crusades and they moved eastward towards what is today Germany and eastern Europe (which is why there are many French words in Yiddish, such as “bentsch” and “lein” and possibly “daven”.
Yiddish dying? – The number of children speaking Yiddish has a primary language has been increasing steadily starting with the late 20th century, and non-Torah Yiddish materials (such as Yiddish newspapers) appear stable. Unless the non-zionist hasidim switch languages, Yiddish is not dying.
Advantages of studying Yiddish (if you aren’t a hasid or a scholar of 19th and 20th century Jewish history) – It would help understand English grammer (which is true of studying any Germanic language, but outside of German the other Germanic languages are significantly less useful to a Jew than Yiddish). While Yiddish grammar is being corrupted by English and Hebrew, Yiddish preserves the complex verb structure also found in English and still has a full set of inflections for gender (though I suspect the “neuter” will drop out under English and Hebrew influences).