Rap may have been around for 20 years, but I don't think it became main stream until the mid 90's. Granted there were a few breakout artists before then. Biz Markie, Run DMC, LL Cool J, Digital Underground, Sirmixalot and Will Smith all come to mind.
I think rap is due for a collapse. Music goes in cycles, and it's time for a Guns n Roses or a Nirvana type of revolution. I agree that MTV plays nothing but crap. Snoop Dogg has gone way down hill since "Deep Cover".