Well Kurt never really broke the mold. He simply mass-publicized the breaking of said mold. There were a ton of singers in Seattle in the grunge scene who had sung with that style before he did.
What grunge did was give people the chance to sing in their own voices. All of a sudden you didnt have to be Motley Crue and sing on the same scale as the average opera singer, you could just kind of... grurable..