In Whole Cake Island we met Katakuri and through him we learnt that haki has levels to it and the more you work with it and push it the better you get at it.
Katakuri could see 5 seconds into the future because of his extraordinary mastery of Observation Haki. After pushing his Kenbunshoku Haki to its peak during his fight against Katakuri, Luffy gained the ability to see into the future to a limited extent. Katakuri himself even noticed that Luffy’s level of Kenbunshoku Haki was reaching his own.
I believe I found a way Luffy can get stronger in a reasonable explanation. I think Luffy will master advanced version of Armament Haki. The next stage for Armament Haki is 24/7 power enchantment kind of deal.
In Brook’s fight against Big Mom there is a part where he manages to get up to Big Mom and stabs her in the chest but it doesn’t do anything, it just ripples the skin a little as if Brook’s sword was just a blunt object pushing down on her skin.
In the assassination plot when the picture is destroyed she falls to her knees and her kids notice that she scrapped her knees off the ground. This shocks them all and it’s a sign in this state Big Mom was vulnerable.
Now with Kaido. He took multiple powerful hits from Luffy’s Gear Third while in his dragon form and transformed back into his human form after being pummeled into the ground, and proceeded to receive a barrage of Gear Fourth hits that were even more powerful, yet he felt nothing more than annoyance, received no injuries, and was able to quickly get back up and counterattack without issue.
I believe this is due to their mastery of Haki. The skin of Kaido and Big Mom is invulnerable. They don’t fear being attacked because nothing can really damage them.
It was said that Kaido has been caught and tortured 18 times, and people as powerful as the Marines and other Yonko have attempted to execute him 40 times only to fail as Kaido survived every single attempt because every single execution weapon broke when being used on him.
This is because his skin is too hard.
Now when Luffy attacked Big Mom she hardened her arm to block him so this either shows she isn’t a master of this like Kaido or she hasn’t recovered from what happened yet.
So how can Luffy use this? I think it’s a case of fight with fire.
Luffy has to learn to master this techingue as well and once he gets to the point where he becomes invulnerable as well he will be able to hit Kaido and actually do something. But this doesn’t mean Luffy is strong enough to beat Kaido, this would just give him a fighting chance.
When Kaido was punched I guess his thought process was “what’s the point in dodging“ he also wanted to test Luffy’s strength to see if he mastered this haki level yet which is why he sighed when he got up.
*Theory by Sniper king ussop