Comparing France linguistically assimilating Occitans with the Rhineland would be like saying that Germany would be quickly able to assimilate all of Poland because they assimilated Northern Low German dialects. Those are different cases.We should note that the objective was not to subjugate the non-francophone peoples and make them second-class citizens, but the reverse: having them learn French would allow them to participate more completely in civic life. French was also the dominant international language in this era so there was an incentive for people to learn it anyway. With the rise of universal public education, I don't see why the people of the Rhineland wouldn't have followed the example of the Alsatians, Basques, Bretons, Provençaux, etc.
Also Alsatian weren't exactly assimilated linguistically, not until the second postwar and even then there were many factors at play that made it easier than otherwise.