Just a thought.
Sasuke and Naruto have as much importance in the 2nd part of the manga, the two have such different path that it beg the question for real: Who was the main character? : Naruto or Sasuke. Kishi answer would be Naruto.
I think the main problem of the manga was that it was both.
Until the pain arc, Naruto was the main character. Until then, the story works well Naruto side. At the end of pain arc, he beat a villain that no one in the village could, as gain admiration from everyone in the village and got hug by Sakura who said thank you to him while Hinata watching the whole thing understandingly and Iruka reminiscing of Naruto path to get there.
When we first read the manga, the hero as like 3 goal
-Become Hokage
-Having Sakura as a girlfriend
-Get respect from everyone in the village.
That ending could have a time skip to show all that happen afterward if it wasn't from one thing and that was Sasuke. Check everypoint. Become Hokage. What was a Hokage in Naruto's mind at 12. The Hokage is the strongest ninja in the village. After pain, Naruto is pretty much that. Getting Sakura could have been made in one conversation and the last point is pretty much done with everyone celebrating Naruto victory. That were the problem was. Kishi wrapped up his main character there. Think about it.
Afterward, the whole war arc revolve around what??. The Uchiha clan, the Sharingan that become the Rinnegan, infinite Tsukuyomi, where Sasuke side (even Tobi was interested in Sasuke for unknown reason that were never revealed) are all part of what drives the whole after pain incident. What Naruto did ? Go to the Kage summit to get the Raikage to forgive Sasuke, fake confession, stop Sasuke, Tame the fox, learn about the past of the beast (setting him up to oppose Sasuke) and fight, losing Neji in the process and then work with Sasuke. His character didn't evolve or advance here. Sasuke, on the other end has all the attention. The kage summit show evil Sasuke at his peak, turned on Sakura, Kakashi and then Naruto. He killed Danzo. Afterward, he got his eternal mangekyou Sharingan and killed Zetsu prime. He confront Itachi and team up to kill Kabuto.(A dead man stop 1 of the main antagonist of the war, so much for the new generation), resurrect Orochimaru to learn more about the for Kage. When he enter the battlefield, it's all about him, which side is he, who trust him, who doesn't. he then talk to the sage until he team up with Naruto and finally fight him to a draw, where he called it a draw to save Naruto from dying of blood loss. 699 is the conclusion of Sasuke's arc. He's the one having to interact with all team 7, ending with Naruto ending him over his leaf headband, saying they're friend again. He's the character that drive this last arc.
So Sasuke was sadly the main character from this point which might explain how the rest kind of suck.