The important thing to remember is that there is still a very strong place for the more traditional models of rap. Rap, much like many other genres, is constantly undergoing an evolution that creates sub-categories of the genre. Gangsta Rap and other styles are evidence of this evolution. Every rapper seems to put their own spin on the genre.
For instance, Jay-Z has a very smooth flow style of rap that was once not appreciated by his peers until he broke through and made a name for himself. Mumble rap is following this same model of being dismissed as not viable but still exploding in popularity.