Oda could have made Luffy <the endurance monster> by puttimg him in front of Hakkai which he didnt. I mean Luffy call this is a big One yet didnt do a shit to block it.. Oda used Zoro there, a valid reason is? Well Zoro is a pure endurance monster (which is something that we already knew from previous arcs).
The current direction of the story would tell you that Zoro is no longer needed on the roof. He achieved massive feats in scarring Kaido, stalling Hakai for a second, protecting Luffy vs 2 yonkou.
His contribution is evident, im also willing to believe that the scar he gave Kaido will act up similarly to Oden scar acting up when the scabbards first attacked Kaido. This will give Luffy an advantage vs Kaido further showing how useful Zoro was.
- Kid and Killer needs to fight Big Mom without receiving too much damage this early on
- Law - needed for support ( the support wouldn't try to stall a massive attack then try teleport the group)
- Luffy - 1 vs 1, no way he would beat Kaido with such injuries this early in the 1vs1...he can't get injured that badly just yet. He'd look before more broken endurance wise than Kaido