I don't really see why ZKK can't happen, especially since Zoro's role isn't quite clear rn. Even if Kaido is defeated and has to be killed later on, Zoro is currently the only swordsman with ACoC (and a possible next PU as well) who can do the job, especially against a very halfdead Kaido. I just don't see why BB or some other third party suddenly appears and kills Kaido, it would be just as random, if not even more.
The only issue is that it's hard imagining how a strawhat kills a villain - well, one of the big antagonists in fact - on panel by beheading him. But at the end, Oda can always bring up something to surprise us either way, not to mention that ZKK theory has some really good core elements.
As an alternative, not related to ZKK, what if Kaido gets basically "sealed" by Luffy punching him at the very depth of the country or later throwing him, with seacuffs, at the huge abyss which Law created via Puncture Wille? Would be quite ironic considering Big Mom fell there too. On top of that, Onigashima could even land there to close the abyss. Considering that it wasn't enough to knock him out or imprisoning him - it always failed - "sealing" him away would be a good alternative against the killing option.