That is the logic fail with you use here.
Yamato is open but Sanji don´t KNOW it. Sanji even fucking don´t know anything of Yamato.
And BM get stopped by Robin/BM, Jack going now down and Sanji as the only strawhats know about the plan.
He going now down to save them, cut of Yamato since Sanji never meet her.
Sanji will go down, if you like it or not, he will face Jack.
@jmena is a Sanji fan, if he even agree that point something going happen for sure.