Board Thread:Naruto Gaiden/@comment-4704473-20150603131227/@comment-945885-20150609114430

Wasn't the sword already inside of the chakra cloak, anyway? Naruto had partially manifested Kurama around the area to protect everyone, and Sasuke was standing within that area when the sword flew out of his hand and into Naruto's gut.

And in The Last, when Naruto countered Toneri's Golden Wheel Reincarnation Explosion, he focused his entire chakra cloak into a shield in the palm of his left hand (then focused his entire cloak into his right hand to deliver the finishing blow).

At any rate, a strong point is made (by Shin, Kurama, and Naruto himself) that the lengthy period of peace has caused their skills to deteriorate. Without having to fight for their lives, even if they do continue to train, it's only natural that their technique would stagnate. In fact, being nearly immortal might even be a detriment, because it means that they can exert less effort to win fights against mere mortals. That's sort of the point of the chapter, and even its title: people evolve through fighting for their lives, and those who stop evolving will die.

I'm reminded of Blade of the Immortal, where Manji's immortality has caused him to become a sloppy fighter, and he often loses limbs or has vital organs damaged while fighting, simply because he can survive and catch the enemy off guard, then heal after the fight is done.

Honestly, based on Shin's scars and all the implanted sharingan, which presumably were transplanted from his "sons," I wouldn't be surprised if he's constantly been fighting his "sons" to the death, replacing his vital organs every time he's mortally wounded. It would be interesting, anyway.