There are dozens of Arguments for Zoro Killing Kaidou
But where & when & how & what Technique will be used ... etc it was mostly just Guesses
If Magma was gonna Kill Big Mom, then the Bombs weren't necessary
And if Big Mom can Survive Magma, Kaidou obviously won't die there
Someone might argue that Bombs were needed to Knock Big Mom Out (Assuming They did) so that She doesn't use her DF to Escape
But if Oda wanted Big Mom & Kaidou to have a Slow Death in Magma while They are Knocked Out & unable to Escape, He wouldn't have added that Massive Volcanic Explosion
It's obvious that Kaidou & Big Mom are meant to Return, and that Explosion sent them far away
ZKK Theory is still possible, and without bringing back all the Arguments, just know that when Zoro achieves his Dream of "Cutting Everything", He must Demonstrate it to the World by Cutting something that is deemed Impossible to Cut
Even if Zoro Cuts the Moon or Red Line or entire Planet, it doesn't mean He can cut anything, it simply means He have Strength to make his Slash that Huge, but to prove you can Cut Anything, you must Cut what the entire World agree cannot be Cut & that's Kaidou
The reason it was deemed Failed Theory was because We thought it might happen in Wano cuz at the time, it seemed like We were in Endgame & that Wano War was Trigger for Final War, but it ended up not being the case, it was more like the Final Arc before Ending rather than Start of Ending
