Also like everyone seems to be forgetting that the "present" demon lord now, has probably been "encouraged" by the bitxh next to him because of her own reasons.
Basically the whole thing just smells like the two top in the Demon army now just does not know what they do but just wanted power.
Also if I were DK I too would have retired there, much more important stuff to do, like being the ambassador to the mc so not all Demons gets erased as well as who would want to rule a kingdom when your orders gets ignored and people no longer believes in you. No way he could have avoided to not incur losses on the demon side after that.