Mate, no one is going downstairs. That doesn't mean that new people won't go upstairs.
Or or or...
We use Occam's Razor and we figure that...
The 5 people who are
ON THE ROOF right now fight the 2 enemies who are also
ON THE ROOF.
King and Queen who are
NOT ON THE ROOF fight Marco and/or Sanji (who needs an opponent because he can't fight Black Maria) who are both
NOT ON THE ROOF as well.
Like, haven't Jinbe and Sanji both already stated that they want to let Luffy go to the roof while they fight the opponents who aren't on the roof since they don't want Luffy wasting his time? Does this not cover King? Why would they allow King to go up? If King decides to fly up, why does Marco allow him? If your answer is queen, why can't Marco ignore Queen and let someone else take over? X-Drake can't fight Queen for a brief moment? Sanji's new opponent can't be Queen?
I'm sorry but you have to understand that Oda has foreshadowed the fight between Zoro and Kaido for the entire arc. The only reason why King vs Zoro was brought in the first place was that King is Kaido's "YC1" (that shit doesn't exist, he is a calamity and nothing else) and because he has a sword (Queen does too). Apart from those two things, King vs Zoro was never hinted at in the story. The reason why certain people still beg for King vs Zoro is that they want to save face because they can't stand Zoro fighting Kaido, it's literally agenda-shattering.