Hey,
i can tell you at least what my problems with ZKK are. You and some people may disagree, but thats fine, its just my opinion. Also, the arguments have no ranking or order.
Powerlevel
Zoro right now, in my opinion, is on YC1 level, he might be slightly above King and Katakuri right now, but still in YC1 range and far away of an Yonko. We see the big difference between YC1 und Yonko by comparison of Katakuri and Kaido, specially when we consider how much Power Ups Luffy already needed to fight Kaido. So i cant imagine to kill Kaido would be in any form honourable for Zoro or anyone, if he is (way) stronger than Zoro.
Luffy takes always the W
Luffy has always been the one taking down the final opponent, specially in a whole arc. He is also fighting Kaido so long and giving that final blow after to Zoro, just makes no sense to me. Zoro had his big opponent and even if i would claim Zoro to be way stronger then King, i know he had his fight already, while Luffy is in his. After all, Luffy is the MC and will take the main credit, doesn’t matter how much we like Zoro, Sanji, Kid, Law etc..
Luffy grew so much already...
In this arc, Luffy got already alot of powerups, from advCoA, CoC, fighting style over all, stamina and finally his awakening. He grew so much to get on Kaidos level to beat him, just to have him being overshadowed after that, by Zoro who got his own upgrades, which was used to beat King. Giving the vice-captain suddenly another upgrade to push him above the MC, which got a HUUUUGEEE powerup now just seems to be wrong, storywise.
Out of Character / Loyality
A very big point in my point of view is Zoros character. He is loyal to Luffy, above everything. He gave away his pride to get stronger for Luffy (his claim to Mihawk), he was ready to give everything and even die for Luffy and he respects him as captain. When Luffy wins his fight or claims this is his fight, Zoro wont interfere. And when Luffy already won and decided to let Kaido live (what he always do with his opponents), Zoro would never ignore the decision of his captain and act against this (indirect) orders. Loyality is a big point for Zoro (look at the Sanji-Situation, or the Usopp-Situation), so there is no way, in my opinion, where he throws away his loyality to gather something unhonorable like killing some already beaten enemy.
ZKK-Fandom...
Its sad that this is an argument already, but it is. If you state somewhere you don’t like the idea or you argue against it, you will get instantly some insults or claims how stupid your argumentation is or how biased you are. A few really argue with you and disagree, which is fine, but the majority just goes to troll you completely, just because you disagree. This point mainly makes me hate the theory. Not because of the theory itself, but because of the fandom putting this on the table on every damn thread. I understand the Zoro fandom like the theory and discussing it is fine and cool, but it got damn out of the hand and if every second thread is about ZKK in some way, its not fun anymore. Specially when another good theories just get lost. Overall, i would love to see more theories and great theorie-threads about different topics.
Like i said, you dont need to agree with me, we can discuss these points, but this is my point of view about this topic and it seems you asked for the reasons. :)
Gz
Panth