In case you haven't noticed, one piece is about pirates and adventures, not pirate revolutionaries.
Blackbeard is the perfect antagonist to end a series about pirates and adventure.
It's not really all that much about pirates and adventure.
Half the crisis for the crew is WG related.
Ennies Lobby - CP9
Thriller Bark - Shichibukai working for WG to capture Luffy
Sabondy - Admiral forcing the crew to separate
Impeldown & Marineford - WG is the enemy and cause of Luffy's brothers death
Elbaf - WG main enemy again
Now the yonkou saga was about pirates sure, but note how it's just a "saga" within the story.
The overarching enemy of the world is the WG.
The reason for the Revo army to exist is cause of the WG.
The ones who made an enemy of Elbaf and Loki is the WG.
Blackbeard has no history with almost anyone beyond Shanks.
Now, If there was still like 1/3rd of the story left and BB had 500 chapters to showcase his World, then sure it could make sense. But with how things are, he wouldn't even reach the throne without an ancient weapon and the One Piece, so even now he's not close to his goal. The manga will end like 50 chapters after he "becomes king".