The big problem with the ZKK theory is that it became an almost cultist movement.
"ZKK" itself and how many users repeated it mindlessly (still remember Finalbeta spamming that ZKK triptych of covers) pretty much evolved into a political motto; sometimes it felt like by questioning the theory you were questioning the identity of those defending it; and let's not forget that proving it false affected its proponents in quite abnormal ways: HAA01 disappeared out of the blue, Finalbeta had a mental breakdown, Cinera yourself seemingly stopped writing your long essays...; and let's not talk about nik87, the walking Dunning-Kruger effect).
As I said many times, the biggest problem with ZKK is that it stopped being just a theory but became exactly that, a cult almost entirely focused on this forum; I can assure you other forums I'm in had it as a delusion and barely discussed it. Some of the arguments could make sense, but the counterarguments were vastly superior, especially:
1) Having Zoro kill Kaidou above the Flower Capital, probably in a double page similar to that of Ryuma, would feel completely out of place for an author that uses euphemisms for "killing" and more or less portrays his characters as the good guys. Zoro isn't doing something so explicitly gruesome which would also rob Luffy the spotlight; also, it was clear from day one his real versus was King as opposed to Sanji fighting Queen.
2) The narrative behind Kaidou being a dragon to be potentially slayed by the second Ryuma isn't as powerful as the actual narrative behind the character, which was, way before his dragon revelation, that of a depressed man bored of life and looking for a place to die who ultimately decided to wait for Joy Boy. This narrative, which again was more important than Ryuma's, was fulfilled when Joy Boy came back and defeated the sad man with a burst of enjoyment.
That's pretty much it. ZKK shouldn't have been more than your average theory and this is exactly how it was treated outside this forum, but for some reason users here went crazy for it and we all know how it ended.