I mean even if Zoro were to outshine Luffy, which probably won't be the case, it wouldn't be the first time he's done it.
Zoro beat the arc villain in his introductory arc.
Zoro outshone Luffy in TB, when he remained standing after taking in all the pain that knocked out Luffy plus his own pain.
Zoro outshone Luffy vs a few of the same opponents they had. Which is minor, but still shows Zoro can outshine Luffy.
Even in this very arc, the early stages of it, Zoro took the role that Luffy usually has, exploring Wano, meeting the arc damsel, seeing the suffering of the people, wanting to free them from it and take care of the big bad, etc.
No idea why that would be so complex to grasp.
Again, I don't necessarily think he will, but saying it like it's impossible also makes no sense.