Marco saved Zoro's ass 2 chapters ago.
You haven't read OP if you think Luffy's pirate king dream against his direct pirate king competitors is going to be supplanted by his subordinate (who has directly subordinated his dream as less important than Luffy's himself, and whose WSSM ultimate goal Mihawk has subordinated his own level as beneath pirate king).
Luffy has more history/drama/ plot/ beef with both Big Mom and Kaido than anyone on the roof. Big Mom and Kaido don't even know who Zoro is, BM came across the sea to chase Luffy, and Kaido was frothing at the mouth at Luffy ignoring him last chapter.