You are basically misreading what is actually there. There are no power levels, so there is no threshold of strength allotted to characters in attack or defense to raise collectively. Roger was killed by regular guys, Kaido and Whitebeard were wounded by people well beneath them.
The ability of Haki/Rokushiki changes many things, but it's skill WITH that Haki, not just strength. Physically there's no reason Luffy would be stronger now than he was in chapter 1000, but his Haki usage is significantly different. Oda uses context like this or unique interactions to dictate. Could Zoro do anything vs Ener in Skypiea? Of course not. Does that mean Luffy would easily destroy Ohm? Fuck no.
No I'm not misreading. You are the one coming back with your word.
Power levels exists.
Zoro could have beated Enel if Oda wasn't inconsistent. He learned haki at Alabasta, no reason to not cut Enel too.
Characters in One Piece are clearly, Yonko Level, Admiral Level, Commander Level, Rookie Level, Fodder Level etc.
But characters have different abilities and qualities. Luffy for example is immune to bullets and punches/kicks/throwns. So no matter how strong you are even stronger than Luffy if you don't know haki or doesn't have cutting/explosion abilities you can't defeat him. That applies to Kaido only being hurt by ACoA and ACoC(that Oda retconned) and BM haki coat that Bege made a whole plan to drop it down.
Whenever you say power levels doesn't exists you are strictly saying someone like Tama can defeat what we beleive to be the strongest or one of the strongest character of One Piece.