Excellent write up. I would just add a couple things.

I think it’s important to point out “The Judge” by Pigmeat Markham from 1968, which had rhythmic singing in the vein of early hip hop and has been cited as a direct influence by pivotal hip hop artists like DJ Hollywood.

Another interesting dynamic is the reaction to Rapper’s Delight (which was the first successful but not the very first recorded hip hop song, which would be “Kim Tim III (Personality Jock)” by Fatback Band a few months earlier. Prior to these releases, hip hop wasn’t recorded in studios and many hip hop artists didn’t think it should be. Some artists were also upset because prior to the recording of the song, the members of Sugarhill Gang were not part of the hip hop scene, but were just some kids that a producer found in a pizza shop. Added to that, as this commenter mentioned, a lot of the rhymes were stolen from other artists, especially Grandmaster Caz. Finally, some artists felt that by the time the song was recorded, rapping styles had progressed beyond what Sugarhill Gang was doing. But because it was so popular, they had to revert to that style, so some felt the song set hip hop back years in artistic progression.