Rock and Rap. They are obviously two different things.
Rock is full of riffs and solos, bass taps and drum fills. Rock used to be an original art that made 4 decades memorable. Many greats have emerged from the pile. Led Zeppelin, Journey, Nirvana, and Pink Floyd. Rock has yet to run its course. Although, nowadays, "rock bands" have completely decimated the reason behind rock. It has no soul. It's just all corporate mumbo jumbo.
Rap....well where do I begin. Rap used to be about some pretty cool stuff. Now it's about drugs, prostitution, and violence. Particularly, revolutionary violence. It's been a well kept secret that rap is the beating war drum for the black community. If you can manage to decifer the lyrics in the songs, you will figure out that it's about hate towards the white and latino population. All these white rappers have no idea what it means, because if they did, they wouldn't be rappers. Rap is just another fad that, if you are starting to pay attention, is starting to run its course.
It will be right there with Techno,Pop,and Jazz in terms of popularity.