Context: WB is talking about the man Roger waits. BB is not trying to be that LOL.
LMAO WB talks shit from his ass about BB not being the man Roger waits of course BB is not the man Roger waits, also WB didn't know BB can steal 2 DF powers.
BB will replace Imu as Xebec wanted to be the King of the World.
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.