Marco’s still going to be the one to beat King. Marco since he’s arrived has been saying he doesn’t know what his role will be. Fighting King will be that role. It’ll be framed as Marco being the Yonko right hand man who has chosen to gamble on and protect the new gen, as opposed to the King, the Yonko right hand man who is trying to kill them. Oda can tie the two of them together as a foil for each other. Marco will throw in some “it’s a new age, I’m not going to let you stomp it out, us two old warhorses can fight it out“ stuff with King while the Strawhats do the rest of the work.
Marco has not been brought into this war to be Zoro’s taxi to Kaido who flies off into the background when the fights start for good. He’s started fighting King, and there’s very little reason to think he’s going to give that up and go do something else- because there isn’t anything else for him to do.