Pure common sense. Final villains = strongest villains. Luffy’s final battle is not going to be a lively debate, it’ll be a punchup. And as the grand finale, the one he’s punching up needs to be the strongest foe he’ll ever face.
And so the two DF using rising power Blackbeard has a huge lead there. Especially over Imu and the Gorosei, who hide behind armies, admirals and assassins.
If the World Gov was the biggest foe in existence then Roger would have teamed up with Rocks to take them down.
Instead it’s the other way around. And Blackbeard has inherited the will of Rocks. He’ll become King of the World where Rocks failed, that puts him directly opposite to Luffy and there’s the final battle. God’s Valley is an obvious preview of what the EoS war will be. The current World Gov will be overthrown in the process and the slaves freed.
The Celestial Dragons are vile, the Celestial Dragons are evil, but the Celestial Dragons are also
weak. They rule because of history, not any of their own achievements.
The World Government is not the enemy, it’s the Celestial Dragons.
Because there’s good people in it, like Vivi, like the Riku‘s. Same for the Marines, good people like Smoker, Coby, Fujitora. And even among the Celestial Dragons there’s potential for change from the likes of Mjosgard.