Luffy is told that Sabo killed Cobra, he doesn’t know what happens. And Akainu killed Ace, not Blackbeard.
And you’re deluding yourself if you think Luffy the man with Joyboys fruit isn’t going to complete Joyboys mission especially after we found out Luffy has a secret dream.
Everything points to imu. Blackbeard really is just a mid saga villain.