Zhao aint done for you are forgetting Riboku behind the scenes.
It's not that easy.
And you just highlighted the problem that could end up for the unification, we don't need Qi now let's just throw it away after everything they have done prior.
Sei understands the problem will be there for the long run, for his goal, Kanki doesn't bother with that.
The unification in the form of Qin can never work in a utopia way
What Haku Ki did was far worse yet Qin attempt unification
What Kanki did was understandably 100.000 if they would regroup under one new general they could annihilate Kankis army letting them go would bring the whole campaign in danger everything they gained all the sacrifice would be for nothing and they would start at zero
Kanki was right in his decision
The original army was 240.000 think about the consequences even if Kanki would team up with Ousen he wouldn't be able to get past that huge army that would be defending
Kanki made a huge stepping stone towards ending Zhao
Even if Riboku would come back that wouldn't change anything Zhao is being reduced to his capital before Gyou Zhao was a mighty state now Zhao lost everything
Between the border of Qin and Gyou
Now between Gyou and Kantan
All those potential Soldiers are now being Qin citizens now Qin can recruit them as soldiers not Zhao
If Riboku comes back he will find the state far weaker than it was ever before
Plus 100.000 highly trained soldiers are missing too
Riboku can't beat Qin anymore