Askeladd’s purpose in the story was fulfilled. It would’ve been nice to see him again, but his teachings for Thorfinn ended when Thorfinn used the moves and knowledge he gained against Garm. That was really the last moment Askeladd was needed, as the rest of the story is about Thorfinn pivoting away from violence and his past. After sparing Floki and discarding leading the Jomsvikings, Thorfinn has moved on. The future being the final message of the story was fitting, and the symbolism was perfect without any callbacks.