It's strange, isn't it?
A Haki attack of Luffy's level actually managed to harm World!
Keep in mind that two years later, during the Dressrosa arc, when Vergo's identity was exposed and he had to silence his witnesses, he fought Smoker.
Vergo had hardened his face with Haki, took a full-power punch from Smoker head-on, yet was completely unharmed. He calmly asked Smoker, "Are you satisfied, Smoker?"
That moment was almost perfectly executed!
In comparison, although World is a Devil Fruit user, he is also a martial arts specialist. Compared to a martial artist like Vergo, World's Haki intensity should be, at the very least, not inferior.
And Smoker, who had already mastered Haki, was definitely far stronger than a Luffy whose Haki was still immature or newly learned.
So why was Vergo able to casually block Smoker's punch, while World was hurt by Luffy and ultimately defeated?
The reason is simple: Haki can only be further strengthened in extreme combat situations. It grows stronger as the enemies faced become stronger.
World, having been frozen for thirty years in Impel Down's Level 6, seemed to have retained his peak strength upon his release, with no apparent regression, but this was only a superficial appearance.
Although his Armament Haki could still harden his entire body, its intensity had severely declined. Against those who could not use Haki, this was naturally overwhelming power. But once he encountered someone who could use Haki, his weakness was exposed.
Otherwise, if it were Vergo with his full-body Armament Haki Hardening fighting the immature-Haki Luffy, Luffy's Haki attacks would have been completely ineffective against Vergo's Haki defense. Excluding the influence of the Protagonist Aura, the result would simply have been Luffy being beaten to death by Vergo.
Therefore, World's severely diminished Armament Haki was naturally no match for Aeridar's powerful Armament Haki.
"How can this be?"
At this moment, World also realized something was wrong. My Armament Haki intensity... it seems to have dropped?!
The realization prompted a growing desire to retreat in World's mind. He wanted to return and thoroughly examine his body. If his feeling was correct, then, as much as he hated to admit it, continuing this fight would surely lead to his defeat.
A scene flashed quickly through his mind. Aeridar's brow arched, and he sneered repeatedly, "Heh heh, trying to run? No chance!"
As soon as the words left his mouth, Aeridar instantly vanished from his spot.
At the same time, World kicked the ground, shattering it and sending a dozen small stone fragments flying. He instantly magnified them tenfold, turning them into pitch-black iron stones that shot out with a pew, pew, pew, carrying the powerful force of bullets.
After the kick, World's figure flashed, and he immediately turned and charged backward.
Bang!
Suddenly, Aeridar appeared in front of World and swept out a ferocious whip kick that tore through the air, striking him squarely in the abdomen.
"Waugh!"
World spat out a mouthful of fresh blood. His internal organs felt as if they had been struck by a heavy hammer, causing excruciating pain. He was instantly sent flying backward with a swoosh, crashing onto the ground dozens of meters away with a boom, the impact kicking up a cloud of dust.
"F-future... Sight..."
Blood overflowed from the corner of World's mouth. He gripped his abdomen, his face pale, and supported himself with three hands on the ground.
"That's your second weakness!"
Aeridar scoffed, looking down with disdain at the disheveled World.
Even though his punches were heavy and his strength immense, to knock out a super pirate with a five-hundred-million bounty from thirty years ago with just one kick was a sign that World's physical conditioning was simply too poor.
It's widely known that Aeridar is an Impact-Impact Fruit user, and his punches are incredibly heavy. Being hit or kicked by him would be unpleasant even for the likes of Whitebeard in his prime.
However, being hurt and being on the verge of collapsing after taking a hit from Aeridar are two different concepts.
Which of the world's top pirates isn't a monster with explosive physical strength? Even Shanks, the only 'normal' person among the Yonko, long ago tempered his body to be incredibly powerful, giving him the confidence to contend with the other Yonko.
As for the others, whether it's Big Mom, who could destroy the giants' homeland at age five and was considered a wicked god by the Giant Elders, or Kaido the Undying, each one is a monster whose pure physical conditioning and durability alone could conquer the world!
The same goes for Newgate, Shiki, and Redfield from World's own era; every single one was much stronger than World. World would have died immediately from an injury like the one Shiki sustained during his one-on-one fight with Garp and Sengoku.
To put it more dramatically, if World had been in the Battle of Edd War, he might have been instantly headshotted by the ship's wheel, which is not an impossibility.
In essence, World relies on his Devil Fruit ability, the accompanying martial arts, and Haki. His physical conditioning and close-quarters combat are his weaknesses. Ranged combat is his true specialty.
On the high seas, World, with the More-More Fruit ability, can transform into a powerful mobile artillery platform, indiscriminately unleashing unparalleled, super-strong firepower on enemies around him, blocking all approaches and destroying all obstacles in his path.
This is why the World Government, the Navy, and other pirates fear him. You'd likely be wiped out by overwhelming firepower before you even got close.
Take the giant cannon on the Groseade: combined with World's More-More Fruit, it can destroy an island with a single shot. It's practically an Ancient Weapon like Pluton, or a modern nuclear warhead. Who wouldn't be terrified of taking a hit from an attack like that?
"Bastard! More-More: 100x Slash!"
World wiped the blood from his mouth, struggled to stand up, and picked up a long blade from the ground. He angrily swung it towards Aeridar from a distance of a thousand meters.
At the same time, his Devil Fruit ability activated. The long blade instantly became gigantic, and its slashing speed was increased a hundredfold. It howled as it cleaved down towards Aeridar's head. The ferocious blade, at this terrifying speed, practically tore the air. Even a Navy Grade 1 battleship would be cut in half by this strike.
"It's useless. My ability counters yours."
A cold smile spread across Aeridar's face. He instantly clenched his fist, and a pale white pressure field enveloped it. He hammered straight ahead.
Crack…
The air cracked open like a mirror, and the giant, high-speed slashing blade was abruptly brought to a standstill, as if blocked by an invisible force.
The terrifying force of vibration was transmitted through the air and acted upon the massive blade in World's hand. Under the vibrating power, the steel blade could not bear the load. Thin cracks appeared at the edge and spread toward the body of the blade. Finally, with a loud crack, it shattered, falling as dozens of metal fragments.
"Pfft~"
At the same time, World was also struck by the intense vibrational force. His internal organs were damaged again, and he immediately coughed up a mist of blood.