zoro will not be sidlined from the second round vs kaido.
if luffy defeated him 1vs1 that'll put luffy on a whole different level from every one in onigashima, even compared to his rivals among the supernova, it'll create a gap in portrayal that's never been created in the series, or any other shonen series.
the mc is always close in strength and portrayal to his rivals, he needs to stay relatable to the rest of the characters.