That’s nonsense. “Fun everyone’s invited” hip hop survived well into the 90s and beyond with the likes of Will Smith, Sir Mix A Lot, Digital Underground, Kid N’ Play, all the way to current acts like Pharrell Williams or Macklemore & Ryan Lewis.
Public Enemy brought their brand of politically charged rap while Run DMC kind of walked the line between party rap and more serious topics. But it was really NWA and Ice-T who ushered in the era of hardcore West Coast gangster rap.
Although technically, three white Jewish guys from New York known as the Beastie Boys were one of the first to rap about sex drugs and violence.
By the 90s, “gansta rap” became the predominant rap subgenre with groups like Dr Dre, Ice Cube, Ice T, Snoop Dogg, Eminem, Notorious BIG, Tupac and Beastie Boys.
Imho, rap was always hard and used as a form of social protest. The softer forms, however, went mainstream first.
Rap originated from the poetry of the Harlem Renassaince: Harlem Renaissance - Wikipedia, and it was also very militant and reflected the Negro experience in America.
There was also a group of Black poets in the 60’s who also wrote very angry poems about their lives.
In other words, militant rap did not just suddenly appear in the 80’s, it was just the first time white people started buying it.