This isn't just about Luffy being the main character but what being One Piece's main character means. Luffy's dream requires topping the rest of supreme kings, according to Chinjao's words; Luffy is the kid-looking guy who was born Children's Day and has a childish dream the Pirate King shared with him; he's more than likely the next Joy Boy, a prophesied comer, and wears a Straw Hat like that guarded by Im among all. And I can guarantee you that whatever Luffy is destined to do, he isn't going to do it with kind words but by force, by winning, by being stronger, because this is a shonen and that's how he solves things.
Again, Zoro can't even dream of being relative to Luffy. Noone in the crew can. This is his story and sooner or later he will top any conqueror out there for sure while Zoro keeps getting the Hatchan, the Mr. 1, the Kaku, the Pica, the King or whatever subordinate significantly weaker and less important than Luffy's adversary.
Whatever "GodTier" the WSS is (and you speak of headcanons; your own terms are made up), it won't be comparable to what Luffy is becoming when this manga is over and we don't post in here ever again.