Whitebeard is outright stating that Blackbeard will not be the one to shoulder the centuries of history.
It's not him. Not his role in the story.
Blackbeard also cannot replace the World Government, for the simplest of reasons being Blackbeard cannot represent the atrocities and tragedies of the 900 years under Tenryuubito rule. He can be evil and terrible in his own right, but he does not have the continuity to claim he represents those damages.
It's like saying if Greenbull replaced Kaido in Wano, the scabbards could fight and beat him to make up for the 20 years of Orochi/Kaido's rule. It doesn't work like that.
WB said Garp is just an old man, and said Shanks is a clown like Buggy, he can say whatever he wants when WB gets butthurt, BB made WB butthurt you are missing the point, BB literally killed him, killed Thatch, captured Ace and caused war, you want him to sweet talk about BB
BB can replace them as he is the only man who can use 2 DFs at the same time and Imu sees him as same level threat as Joy Boy and Ancient Weapon while Imu himself being a DF user, who even Doflamingo thought he could beat Imu with Ope Ope fruit. Oda also said story is based on fighting vs Yonko and after finding One Piece the story will finish so the final battle is vs. a pirate not vs. Imu who is not searching One Piece.
Keep crying.