You purposefully ignore foreshadowing if you think Shanks and Teach won’t fight lol. It’s been the most abundantly clear thing in the manga besides Luffy becoming pirate king.
Why would Oda reduce Luffy's opponents? You didn't answer.
He is already doing it via Law and Kid, they made them fight Bb and Shanks. And you want Shanks fight BB too after? Lol.