Kakazu is a beast for sure but Everyone who fought kakazu was a fodder excluding kakashi. [kakashi didn't even use his MS]
Naruto comes and Solo's a weakened kakazu at the end in base form.
Imagine Naruto in sage mode? Overkill.
Naruto was using Kyuubi chakra, which has been shown to make him strong enough to stop giant summons, fast enough to hit Haku, makes him heal faster, and increases his chakra. So no, he wasn't in base form.
Naruto also didn't "solo" Kakuzu. Kakuzu first of all, as you said, was already weakened due to his battle. Kakashi ambushed him and destroyed one of his hearts, reducing his power by 1/5th. Then Hidan destroyed another heart, reducing him against to 2/5ths. Then Naruto and Yamato had to save Kakashi, Ino, and Chouji. Then they told Naruto a summary of Kakuzu's abilities and needed Kakashi's and Yamato's help.
Now imagine Naruto fighting a 100% Kakuzu? I would say he'd need Sage Mode to win.
Kakuzu:
Very intelligent
Very strong
Very fast
Very skilled and experienced
Five+ lives
Self-reconstruction
Nearly indestructible body
Doesn't breathe
Nearly immune to poison
Immune to genjutsu
Deadly tendrils
Masked Ghosts
Five elements
AoE attacks
Shared vision
But Naruto vs Kakuzu was nothing like the example you stated. Yes Kakuzu had lost some of his hearts, but he still had, i believe, three left. I don't understand how you can say Kakuzu lost by plot when the whole of Naruto is plot. That is like sayih anyone who has ever lost a battle was due to plot. Like, Third Hokage didn't lose to Orochimaru, that was just plot. Konan didn't lose to Obito that was plot. Five Kage didn't lose to Madara because That was plot.
Kakuzu did fall for Naruto's plan with his experience otherwise he would have figured it. Canon better than your opinions.
With each heart Kakuzu lost, he lost a lot of power.
When he lost his water heart first, he lost a couple useful elemental combinations such as spraying the area with water and electrifying it, or shooting water and fire at the same time to cover the area in a boiling hot mist, etc. Not to mention that he also lost one of his Masked Ghosts, bringing him down to three that he could use in battle... and they had a hard time dealing with just those three.
Then when he lost his earth heart, he lost his ability to turn his body to adamant or travel underground, as well as losing another Masked Ghost. With Domu, his body becomes as hard as diamond and not only increases his defense, possibly being able to resist Rasenshuriken like how 3rd Raikage's steel-like skin could, but it also increased his offense; a punch from him could bust down a sealed iron gate. He couldn't use that against Naruto.
When people say that Kakuzu lost by plot, they mean he didn't lose in a believable way. He went from picking at Shikamaru's brain to falling for the oldest trick in the book, twice. Not only that, but he conveniently forgot that Shadow Clones use jutsus too and he didn't hear the Rasenshuriken's screeching sound coming from behind him.