Since you want to argue, we can. I did not ignore Marvin Gaye to make my argument stronger. Marvin Gaye died well before Chris Brown was born and Beyonce and Usher were toddlers, so the idea that the music bridges a long period of time still applies. Furthermore, Marvin Gaye was a pop artist. Think about all of the singles he released for Tamla prior to gaining artistic freedom in the early 1970's. Even after he gained freedom and started multi-tracking his voice, he still turned out a bunch of great pop like After the Dance, Got to Give It Up, and Sexual Healing. As for Fats Waller, you are the first person I have ever known of who called his music r&b. Most critics I know of considered him part of the jazz or the swing era since his peak was in the 1930's, well before early versions of r&b like jump blues were created.
As for Lawrence Cohn's opinion, I respect it, as I respect yours. I don't, however, agree with it. I think that r&b is a very broad category like rock or country, and over the years, there have been very distinct movements within the genre like Soul, Funk, and Neo-Soul. I will agree with Cohn to a degree that r&b is a bit of a catch-all term, but I feel that popular Black music is distinctive enough that it deserves its own category.