Exactly, You fail to see why.
I think this discussion is pointless. ZKK might not happen, but if it does, i have to believe Oda will handle it in a way (maybe a totally unexpected one) that doesn't take away anything from both characters.
You are free to believe whatever you want of course, and you could be right in the end, but i personally can't ignore all the plotlines related to Zoro. Again, I'm perfectly aware that those plotlines might not even be fully resolved or resolved the way I expect them to be.