I think only the arguments against ZKK are not that good given all the supposed hints/foreshadows that the theory has in support of it. The argument of Luffy being the MC holds the most weight in my eyes out of all the other ones but we also have that comment of the ending of Wano being surprising and this to me is the biggest reason for discounting this notion of Luffy defeating Kaido just because he is the MC. We have the established storyline of Luffy defeating the main villain and everything depending on Luffy defeating the main villain solo and then Luffy liberates the village/island/country. If we remove the ending of Wano being surprising comment then this would mean that the arc will follow the established structure.
I think if we go stepwise we have the first villain (don't know if we can count this one as such but anyway) Alvida and Luffy defeats her and frees Coby to follow his dream. Then we have Morgan who is defeated by Zoro but it is Luffy who is actually responsible for the town/island being freed from Morgan's rule, had Luffy not saved Zoro and helped him, Zoro would have died and the town would still be under Morgan's control. There is no indication though that Luffy would have lost to Morgan at all. Then we have Buggy and Luffy defeats him and frees/saves the town. Then we have Kuro and Luffy defeats him and save the island. Usopp here defeats Jango (who is the VC/First Mate/Acting Captain of the group though he is not the second strongest). Then we have Don Kreig and Luffy defeats him and saves? the Baratie, Sanji loses to Gin in battle I think but wins due to his kindness/values. Then we have Arlong who Luffy defeats and frees the island, Nami did not beat any officer of Arlong Pirates IIRC. Then we have Mr.3 who was defeated by Luffy though there is no saving of the general public at Little Garden. Then we have Wapol who Luffy defeats and his henchmen/ministers are defeated by Chopper, Luffy saves the island. Then we have Alabasta where Luffy defeats Croc and saves the country. Then we have Skypiea where Luffy defeats Enel and saves the island/country. Then we have Enies Lobby where Luffy defeats Lucci and saves Robin (primarily), Franky defeats a lower level sub-ordinate, Robin got her revenge on Spandam. Then we have TB where Luffy defeats Moria and Oars (with the help of the crew) and saves everyone but then Zoro sacrifices to Kuma and saves Luffy and potentially the others too (there is no indication though that Kuma had any interest in anyone besides Luffy at that time though), Brook saved Robin and Franky earlier then helps in defeating Oars.
Time Skip
Now we have Hody and Decken and Luffy defeats them and saves the kingdom, Jinbe gives Luffy blood and defeats Wadatsumi together with Sanji (a development of a duo begins I think). Then we have Punk Hazard and Luffy defeats Ceasar and saves everyone (no indication though that others could also not have defeated Ceasar), Law defeats Vergo (I think he was stronger than Ceasar). Then we have Dressrosa where Luffy beats Doffy with Law's help and saves the country. Then we have WCI, Luffy defeats Cracker and Kata and rescues Sanji, Sanji saves everyone by defeating BM with his cooking, Jinbe covers the escape of the SHPs.
And now we are at Wano and there are things which are somewhat different from the previous arcs. First of all there are two threats of similar power level, Kaido and BM, although beating/killing Kaido is the main objective. We have a character like Jango and Spandam in Orochi who is himself not that powerful but has authority. The problem here is that Kaido is not like the previous villains, he is the strongest creature, the gorosei are afraid of angering him. All the previous villains could be controlled or kept in check by people other than Luffy since there were other more powerful people especially in the marines.
So this leads me to believe that Kaido must die or we have some sort of Nagato moment and Kaido becomes a saint all of a sudden (can't see that happening at all). Killing Kaido, taking Kaido's head, cutting his neck has been repeated multiple times in the arc. Kaido being unkillable has been repeated multiple times. And if Kaido was just some brute who lived in Wano then we could accept that he might leave Wano after his defeat but Kaido specifically chose Wano and to drive home the fact the guy is flying a whole island and is going to land it on the previous capital of the country. I think that Kaido must die in this arc, how it is achieved only Oda and a select few know. The surprising ending might even be Luffy killing Kaido himself which would be a stark contrast to what has been established previously in the story. The good guys/SHPs not killing anyone is just a moot point, Zoro had a reputation for it, Robin is an assassin and Luffy has done things to fodder that would have killed we just never got confirmation (he did ask Zoro to save Smoker though) but then Luffy can tell whether someone is good or bad.
Defeating Kaido is also somewhat tied to the 20 years prophecy in some way since Oden believed or knew that no one else would be able to defeat Kaido for that time.
Only Zoro has defeated the main villain of his joining arc, Usopp and Chopper defeated the right hands/vice-captains etc., Franky and Jinbe defeated a lower level member, Brook helped in defeating Oars (don't know where to place him in the hierarchy), Nami and Robin didn't (Robin did break Spandam's spine but it was not like the usual battle) and Sanji lost his battle but won his war you can say. I am not talking about what could have been had this or that happened but what actually happened (don't come after me like I am a Sanji hater or anything).
To summarize my points I would say that killing Kaido is pretty important (he is not someone you can treat like previous villains). The ending of Wano is going to be surprising so it is not going to follow the previously established formula for arcs. There is precedence of Zoro defeating an arc/town/village's main villain (though it does not say that Luffy could not have done that etc., this is not a power level feat). There is precedence of Zoro doing something at the end of the arc in which Luffy is unconcious (it depends on how people perceived whether Zoro out shown Luffy in TB or not). Zoro has tried to and killed people on screen (Mr. 7, Orochi, CD and the magistrate are examples).
I think that there is a good chance of ZKK happening (in what manner idk) and I would most probably like it (depends on the manner in which it happens). If it doesn't happen then I would a little disappointed since it would feel like a wasted opportunity for Zoro to start cementing his legacy as the future WSS but I trust Oda and so far I have enjoyed OP. IIRC Oda did say that he writes the story/manga with a casual 15 year old boy in his mind as the primary reader and he does not see to many youtube videos etc about theories and how deep some of the people are reading and analyzing his manga or the theories that are coming up based on the hints cause otherwise it would somehow affect his storytelling thinking that his readers must know that this thing is sort of a given based on the previous stuff and he would not draw or tell some of the things that will confuse his primary reader but will be pretty clear to the theorists and deep readers etc.