Luffy declared he can't be pirate king without Sanji. Not without his cooking, without HIM.
This is illustrated by the fact that Luffy asked Sanji to join BEFORE he ever tasted his food. Wanting him for him, and his role only second. A very symbolic difference when it came back in WCI with Luffy's statement (and why I hate the Live Action for changing it,)
Thus Sanji could never be replaced because even to Luffy, Sanji's cooking comes second to Sanji himself.