They can become the strongest beings in the verse yet BM and Kaido are mostly failed as characters and their stories presence aura personalities etc are a lot weaker than Shanks and WB
Failed in what context? They are on the same playing field as the monsters like Shanks, Whitebeard, and Roger. Kaido is superior to Oden who was world renowned in his own right.

Both Big Mom and Kaido have developed their own armada, military might, and global influence. Their individual movements are monitored and tracked by the World Government. Legends like Garp and Sengoku fear what terrorism they can bring. And they did this by moving on up in the world the same way Shanks did. They were also crewmembers of a legendary Pirate.

The only people I might give a slide to are Whitebeard and Roger who were rivals and both King of the Seas.
If you're talking about Big Mom and Kaido failing in their dreams, then I understand. But then we have millions of those on the sea like that, except they don't have anywhere near as a reputable or feared name.
 
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