He literally was actually lol, 99.9% of the OP world called him a Yonko after defeating Katakuri.
It was not just defeating Kata that made him called the 5th emperor. It was may things listed by Morgans, as having a huge army, going in WCI, destroying an emperors place, defeating 2 commanders, and all the credit given to him which is mostly true.
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Katakuri has CoC and will take over the Big Mom Pirates. Marco has no CoC, and he couldn't hold WBPs together.
Katakuri in fact will be the captain of Big Mom pirates, yet he wont have the same influence as her. I mean, it will be just a strong crew but with no emperor tier feats. Maybe they will lose control over some islands, the gyojin island being one of them, they wont have the power to be a candidate for Pirate King.
It will be crew tier below the emperors.
But seeing how Law doesnt have that much military power, there is still the posibility.