It's tantamount to ethnic cleansing. The Rhineland was one of the most population-dense places in Europe even in the early 1800s and a near absolute of them were German/German Speaking. To have been reduced to a minority language in simply a century implies more than just a few language policies and their consequences.
IRL, Corsican went from having a 85% transmission rate to a 30% one in the span of 20 years between 1914 and 1934;
Many studies of turn of the century/early 20th french languages report that families where the younger siblings couldn’t speak the local language while the older sibling of the same familly could were a common occurrences.
Transmission breakdown can happen very quickly within a generation. And once there is a transmission breakdown you just have to wait a lifetime to see an extremely quick collapse of speakers of a language, this has affected many languages and still affects many languages, the first generation after a quick transmission breakdown usually keeps a good level of passive understanding but is rarely comfortable with using their ancestral language for everyday life’s
IRL quick Gallicisation affected most peripherical french cities uniformly (with some exceptions) from the start of the third republic and urban inter generational transmission of local languages had mostly stopped by 1900, the Rhineland being very urbanised, and considering TFS France was more politically stable and developped. I think it’s plausible such transmissions breakdown had been accomplished in the Rhineland by 1900, add 40 years and the working population is mostly francophone.