You know, Oda can very well make Queen’s arm work as a flamethrower too. I have never said that Sanji wil take King down alone, but leaving a fight to Zoro and say “I am too weak for him, he is yours” is strictly against his character. Sanji and Zoro never intervened with each other’s opponents, unless they work as a tag-team (I am completely okay with 2v1 or 2v2 and would be more than satisfied). 2v1 with Marco against King is also a good option with Sanji acting as a support or main fighter in the fight, since Marco is needed against Yonkos too.
King, unlike Vergo, Doflamingo or Daifuku, attacked directly with intent to kill, from which there is no turning back. Sanji has no excuses but to fight him at this stage.