First off, I agree that the focus on saving Sasuke really hurt this story. When you set a premise like that so early on in your tale you generally back yourself into a corner of having to write a redemptive story about the guy who needs saving while focusing less on the guy who helps save him. We were drawn in early by the Naruto character and his tale of rising up from nothing and working hard to attain your goals. Then at the very end of pt1 we have the groundwork laid out for the redemption story. That's what happened to this manga. I don't think Kishi quit caring until the very end, I just think he genuinely never thought out how he could write this idea of his well.
That's why everything feels so forced. He refused to give up on the redemption of Sasuke. So he felt he had to chronicle Sasuke's descent just as much as he showed Naruto's growth. But to make people relate to Sasuke more he felt he had to write a legitimate and detailed backstory that would cause readers to feel sorry for Sasuke and root for his redemption. Unintentionally creating a lack of backstory concerning the MC and his family, along with other characters. This wound up being hit and miss because Kishi isn't a good enough writer to get the majority of his fans to understand what he was trying to convey. Some bought it and some didn't, yet all the excuses he wrote into Sasuke and his family's background wound up feeling so lacking or contrived. Those writing skills are what split the fandom in so many ways.
His refusal to make Sasuke his antagonist to Naruto's protagonist, or at least rival who acknowledged Naruto somewhat as often as he was acknowledged, left Naruto without a villain who could maintain focus on the MC. Therefore maintain focus on the actual plot. Instead he kept Sasuke as someone who was indifferent to the MC and caused a lessening of the MC's importance in the eyes of a lot of fans. Naruto just didn't come off as cool because he was the one constantly thinking, worrying and chasing after someone who hardly gave him a second thought. This in turn allowed fans to start caring for or concentrating more on certain side characters and the pairings.
Kishi was just not capable of writing a story as complex as the idea he had. One of a hero's rise while simultaneously redeeming his rival who supposedly never completely descends into the darkness in a long drawn out fashion.