Zoro himself said he wanted to avenge Yasu, Zoro and Orochi plotlines were coming nearer and nearer (Kamazo, Hiyori, Yasu, ...) in front of our very eyes.
The fact that Oda seems to be following a different route, it's not our fault. You should blame Oda for being a third class writer.
Zoro vs Orochi wasn't necessary, but a panel similar to this with Zoro instead of Shinobu would have been the bare minimum to end that plotline.