ZKK STILL HAPPENING!
Zoro isn't out yet.
1. He still has to make Enma black (this clearly has to happen in Wano)
2. He has to do something to justify a massive bounty increase (the rooftop is not being streamed to Marine HQ)
3. There are a lot of chapters left, presumably one of which will have a name like "Roronoa Zoro"
Zoro is not fighting King.
1. No interaction between these two characters
2. King was created to fight Marco, given their devil fruits
3. The alliance has more combatants that need a fight (Yamato, Sanji, Scabbards) than there are viable opponents
4. Given his current state, in order to fight a calamity he would need to recover first, but if he recovers it would make more sense that he return to the rooftop
Luffy is not soloing Kaido.
1. Would make him the strongest character in a story that is going to take another 5 years to finish
2. Would make dozens of characters (Worst-Gen, admirals, Sanji...) borderline irrelevant
3. The power scaling implications of such a scenario are nonsensical, even for Oda (who has acknowledged this himself)