In Wano, the story was about Momo and Kaido, with Luffy always playing the middle man.
In Fishman Island we had duel villains in Hody and Vander, so we got two arc characters in Jinbe and Shirahoshi, with Luffy playing the middle man.
In Enies Lobby the villains were Lucci and Spandam, with the 2 arc characters being Robin and Franky. Luffy always plays the middle man connecting everyone thematically.
I mean you can say WCI is a dual adventure aswell, one side is getting Sanji back the other is stealing the Poneglyph, those are the 2 main objectives. Pedro is the main relevant SH "helper" of the arc to do that , when he died, his narrative succesor is the Bunny, making it a "real time" sad back story.
That's why i said WCI is kinda of a clusterfck.
Also here's the thing, so then now what? Are we going to get a new princess of Elbaf that's a giant with a massive rack that is related to the villain of the story ( unlikey since we need Yonko villains now) ?
Is that the new SH? Are we going to make Elbaf a Caribou or Bonney centric arc? Is smoker going to sudenly appear and make it about him? Or Boa? Or whoever someone brings up?
That's the issue, the next Yamato/Carrot/ Rebeca/ whoever is right around the corner, that's whos going to get focus, now you can either get a better character than those Yama/Carrot, or just as bad. Regardless you're just kicking the ball further and further down the line and closer to the end of the story.
I makes more sense for Yamato to unironically come back than them getting anyone new.