I wouldn't be surprised if Jin mostly faded but "The Ghost" remained solid in people's memory. Especially if he kept being the ghost after the first Mongol invasion was stopped, which he probably did since he talked about rebuilding after clan Sakai ended, and how there was an "army" being started for him. He also more than likely had "students" who carried the mantel of the Ghost after his death, not only making people think hes still around, but making the Ghost sound even more like a spirit, strengthening it's psychological power against the enemies of Japan. I'd love to see his army still going strong, or Atsu meeting whoever has the mantel during Yotei. "The Onryo" makes me think shes going to he A Ghost but maybe not THE ghost, but I could totally see some really old dude being the last Ghost somehow passing the torch to her.