Losing an arm is not exactly a new concept in anime but many have still managed to be pretty bad ass despite that. Do you remember how orochimaru was with those arms of his in necrotic state? Even with barely any use of his arms he was still able to wipe the floor with the other two sanin until Tsunade was forced to break her seal. Even then they still had a real pain of a time trying to beat him even if he had cameltoe oops I mean kabuto with him. Minato may have one or two tricks left but in terms of raw power naruto is pretty much outclassed him in that department now that he has HBM.