The real reason why Marco is rather obviously losing in this battle is that he's already using his hybrid form, and yet neither King nor Queen have brought theirs out yet.
Thinking about it another way, obviously we need to see Marco have a little shine and get a badass moment or two in this arc due to him being introduced. If we didn't, do you know the sheer number of people who would claim on those grounds that Zoro & Sanji are way above Marco?
The ultimate plot really couldn't be more obvious. Marco will manage to deal some semi-significant damage to the calamities, then his regeneration will run out, he'll be defeated, maybe even killed, by King and Queen bringing out their hybrid forms (which they've been hesitant to do since they're confident in their abilities and not wrecking Kaido's house is still a priority to them) and Zoro/Sanji will be there to have their arc fights, finishing off the nerfed calamities.