The ending is just bad, the author can clearly make them still a demon, being a human doesn't solve anything.
A new world? She need that fuckin demon lord power to rebuild civilization from scratch, do you fuckin think civilization built in a day? They're going to live a primitive life for the rest of their live and their peace won't ever be reached after they died, they should start at the middle instead of literal 0
It is unlikely that all civilization died, she changed the rules of the world. So the world got rewritten, judging that all of them are alive, likely everyone is alive. But she can't prove that, hence she said let's go find civilization and if not then we will make one
How is that the ending? She has the power to quite literally do anything and instead of maybe making peace for the world or something else between races, she thinks the best route is to literally destroy everything and make everyone human? It's not like humanity was portrayed in the best light in the series from the little we saw of it, does she think that just because everyone's human now there won't be awful consequences down the line?
This feels like a real poor attempt to wrap things up even though it's been going at a fast pace for a while so I can't quite blame the axe and there's no light novel either, I guess they just wanted to wrap things up with a meh ending.
She gets only 1 wish, how would she make peace for the world? The world is forced into battle by the rules of the world. She destroyed the rules so the world was rewritten. She didn't think everyone will be made human, she had no clue what would happen. Likely demons are just humans and animals that got warped by mana. So by destroying the rules it just ended up the end result
In the first place there likely is some limit to the wish thing. She isn't the first demon lord, likely every demon lord had a wish granted. Yet why aren't humans now cattle to the demons? Why isn't the human realm conquered? The likeliness is that the rules end up warping wishes. Like the 2nd demon lord likely wished for no heroes to be born, only for humans to make artificial heroes because the rules likely restore the world back to conflict and balance.
It's like imagine a super AI, and you get to add 1 line of code to it. How confident are you that in a single line of code you can fix it without it finding a way around?
As frustrating as it is, destroying it is the only right answer