Oda introduced Zoro's plot-lines and abandoned them.
I personally never propagated ZKK here like a lot did, but I did however criticize the decision to not resolve any of the plot-lines.
And no, it has nothing to do with power stuff, either. Oda could make Zoro weaker than what he ended up as, but actually wrap up the ongoing story and character arc he had in the Wano, and it'd be a far better ending.
Regardless, the notion that a side protag might kill a main villain of arc (just an arc, not series) is nothing weird. It happens often in Shonen and fanbases usually love it. One Piece fanbase (or at least Luffy fans) are only ones I've ever seen be so insecure about just the notion of it lol.