We have to look at the thematics of the story, and everything leading up to Wano. Oda kept hammering home the point about taking down Kaido and Orochi. Even though Kaido was the focus of being the Big Bad, Oda still placed heavy emphasis on Orochi needing to be defeated.
Wano might be under Kaido's control from the shadows, but the people of the Country recognize Orochi as their leader, and one of the key factors of their suffering for all these years.
Even though Kaido is also a factor into the suffering of the people of Wano, the two actually share this sentiment. The two are basically two sides of the same coin. The people need to witness Kaido's defeat, but at the same time, the way Oda has setup Orochi, it is also clear that the people of the Country also need to witness Orochi's defeat.
That is the ONLY way the horrendous chapter of the lives of the people of Wano can come to a close. It can't be one or the other, but rather needs to be both, as this is the first time Oda has had two antagonists be responsible for the suffering of one Country.
There is no way in hell Oda can throw all that out the window, and turns this strictly into the Kaido and Beast Pirates show. It resolves nothing on Orochi's end, and would be a terrible way for Oda to go about concluding Wano. The best way Oda can rectify this, is by turning Orochi into something that needs to be dealt with, and witnessed by everyone in the Country. Only then can they truly be free from the shackles that has bound them for over 20 years.