Luffy says a lot of things. Blackbeard will defeat Shanks, I definitely think you agree how hard the build up is pushing for that, right?
Or do you earnestly think that luffy will have an all out battle with his idol?
None of these story lines are that important in the grand scheme. Oda has said a million times recently how he wants to finish OP as fast as he can, his editors confirmed that they've been shook by the pace of Wano meaning Oda sacrificed plot points in this arc to push the story forward, same will happen going forward.
Big mom and Kaido won't matter in the throne wars post this arc. One will die and the other won't be an enemy any longer.