Many thanks
Honestly Schwartz makes no sense. Like why does it matter who is the demon king and Kohana:s progress to become a hero. Like who is to say the world is ending. Or whatever thing Oracle they have is the only way to not getting the world destroyed.... Geez
I think the general idea is:
It is known that following the route of the original story leads to the good ending of the original story. (low risk, medium reward)
It is unknown whether following some other route leads to a good or bad ending. (high risk, high reward)
Schwarz would rather have the story play out, tragedies included, to guarantee a good ending, rather than preventing tragedies and entering unknown territory.
It makes sense as a motive.
Of course, they are villains. Their motive might be flawed.
But it makes enough sense to not be dismissed out of hand, which is why Iana struggles with it.
We have not seen the world-ending threat yet. Maybe in the story, if Konoha had been any weaker she would have lost, so every bit of trauma that would make her stronger matters.