I’m not saying they didn’t beat Big Mom.
I’m saying that it was good writing for the World Gov to hold off on making Blackbeard a Yonko, despite what he did in Impel Down and Marineford, because the title is more than just “strong pirate” and it required a cementing of his position.
The World Gov should be aware that
-one vs one, Kid and Law have zero chance against a Yonko. They don’t even put up much of a fight
-they don’t have any impressive manpower
-the only strong ally they have is Luffy, who is certainly going to be an Emperor in his own right
-they have no real territory under their belts
If they make Kid and Law a Yonko now then it’s a bad joke. Look at how the Yonko have been portrayed so far. Pirates with the power to make the entire World Gov step warily, bring in all their forces, need huge alliances to take on.
The Kid Pirates and the Heart Pirates could be defeated by a single top tier.