luffy led a part of strawhats to wano. zoro led a part of strawhats to wano.
both of them caused trouble when they came.
both fought hawkins.
luffy has a connection with tama. zoro has a connection with otoko.
luffy has a connection with okobore town. zoro has a connection with ebisu town.
luffy has a connection with hyogoro, a man from old wano. zoro formed one with yasuie, a man from old wano.
luffy met kid in act 2. zoro met killer in act 2.
luffy learnt ryou to fight kaido. zoro got enma to fight kaido.
luffy had a character moment with momo. zoro had a similar one with hiyori at yasuie's execution.
even narratively, we explored wano with two main plot narratives, one revolving around luffy's adventures while the other revolved around zoro's. the other plotlines were effectively tied into either luffy's or zoro's.
plus out of all the strawhats, only they've said it explicitly that they're gonna fight kaido in this arc.
and kaido has commented on both of their conquerors haki.
if you see luffy + zoro's characters make up oden's character. a part of luffy's and a part of zoro's.
you can't just put sanji up there like that when there's so much buildup for these two to fight kaido.
i can totally see luffy 1v1 kaido, momotaro happening. but there's a good chance that zoro goes up as well.
Or now with that knowledge Killer can just attack bodyparts of Hawkins which are replaced by kid anyway. For example the arm. Permanently hit the arm, open all veins etc., and nothing happens to kid, since doesn’t have that arm anymore.
I have to respect Kid. His durability might actually be better than that of Luffy.
He tanked a Yonko punch to the face, tanked Apoos attack, freshest one out of the rooftop battle and now withstood Hawkins voodoo attacks that have canonically ko everyone.