We've seen techniques being used to bypass durability, but even so KAIDO is so hard that the damage doesn't penetrate any further than the surface of your skin.
A technique to ignore durability, are you aware of how effective this technique is on the other layers that have different skin from KAIDO?
So it is.
To defeat another upper layer you don't necessarily need the same AP to defeat KAIDO, know this, a move that fails on KAIDO will not fail on another opponent X, understand that.
KAIDO can withstand greater amounts of injuries and attacks thanks to his recovery, he doesn't suffer the consequences, the others are suffering the consequences for the smallest of blows.
Whitebeard was suffering the aftermath of a small stab, Katakuri and Luffy was suffering the aftermath of a stab, G4 luffy's punches with ACOA in FUJITORA will be extremely damaging.
Luffy only needs to increase his AP more and more to beat KAIDO because KAIDO recovers from his hits that cause damage but recoverable, KAIDO is fine from injuries, but the same treatment will not be done for another top layer, because his blows will work effect, they will hurt him and the wound will remain, understand?
So Luffy will only need to keep hitting hits that have effects, until Admiral X can no longer resist.
About your comment, I don't consider WB attacks as lethal, people are impressed by the fact that he was able to split half of the island, if ZORO tries he can split half of ONIGASHIMA with ASHURA the difference is that ZORO's coup is reaching and leaving a scar on something more durable than AKAINU.
Whitebeard, he had his HAKIS in degradation, he didn't have an ACOA, an ADCOC, to boost his AP it was purely his DF and a COA,
Whitebeard's attacks were treated with exaggeration, even on the navy giants, Luffy took down a giant with 1 punch in DR.
Whitebeard punched the navy giant that his shock wave had to be contained by the 3 admirals to not hit the platform, for luffy to do the same doesn't require more than a punch on the g2.
The exaggerated attacks of the WB is why people do not understand how your DF works.
ZORO's cut with Flying dragon Blaze covered 50% of the island and cut ONIGASHIMA's horn, this targeted blow on the island could do something similar to what WB did, in addition, AKAINU only resisted 2 shots from WB, he was incapacitated for chapters and only appeared later.