I hope we won't have immortality outside of Imu. It should be once every 1000 years type thing. I think the gorosei are just "special" like giants who can live 300+ years. And yeah, Dragon killing one of the gorosei adds up, just don't think it's Oda's style unfortunately. He won't have one of the "good" guys kill people. At least not explicitly. If he dies, it'll be Blackbeard swooping in.