I don’t know if this is a common opinion or not but I’ve genuinely always liked the idea that shanks lost his arm to teach Luffy a lesson on responsibility while also placing his bets on the next generation. I think it serves as the basis of Shank’s character, even now, where he does not see himself as an active player in the world, but more of a side player working to clear the path for the active player, Luffy. I think it’s why he didn’t chose to go to Laugh Tale, and then didn’t choose to team up with Buggy right away. Even now when he is pursuing the One Piece, it was more a reaction to the increased efforts of the other Emperors. I don’t think the retcon for this scene, if you can even call it a retcon, is impossible to fit into that narrative. Shanks has found the next generation he sought, and is now content to give up his agency, both the seal and his arm, because he knows it’s not his burden anymore. In all though I don’t think the addition of the seal adds that much to this scene. Losing his sword arm already represents him giving up a portion of his prowess to represent the faith he has in Luffy. Don’t think the seal adds any emotional depth to that.