I wrote an essay but fuck it.
Sanji went in badly wounded against vergo and he wasn't struggling much when he was blocking a gigantic genie on sea. Him getting snuck and blown back by daifuku means nothing. It happens to luffy all the time too, I could act like he struggled against ulti, or say zoro was struggling against apoo. Just disingenuous. The all stars also all have the same title. The beast pirates hierarchy is dependent on strength but queen and king hold the same position so your point is pointless.
Marco isn't fighting the main antagonist under the dome. He is for one, not relivent enough to get a full 1v1 against the right hand, two, he is simply to strong, King would not get properly showcased and that was made clear when Marco took both Queen and king on, there would be no stakes in this matchup becausethe outcome is so boringly obvious. And king needs hype, he needs to properly dunk on someone, make someone bleed for real. That's not gona be Marco. That will likely be sanji. We need someone to struggle against King, who is precived as weaker only to get revealed as stronger at the end of the day. In that sense not only does sanji deserve King but king deserves sanji because the showcase he would get out of sanji is much much better. Marco isn't getting this fight, we have main characters for a reason. If it's not zoro, it's sanji. Also, not believing in the ridiculous powerups of the m3 at this point in the story is hilarious. Sanji can get new forms, new dj variations, he even has one that was teased against judge but never used, new observation haki awakenings and a proper showcase of his suit.
Sanji can become exactly as strong as he needs to here, making some bs strength argument and saying he can beat queen but not King who isn't even that much stronger from what we know is bullshit.