The author had personal issues which forced them to rush the ending guys. I'm sure the plan was for them to fully figure out the human demon problems but this is about as close as we'll get.
This manga never delved all that deep into the human side of politics because it focuses on characters who were never actually involved in that. Hell we don't even know the Prince's political standing, maybe he was a puppet leader and him loving his wives and family was his way of coping. If that's the case, getting involved with a demon caused the actual political powers to view him as a threat and choose to eliminate him under the excuse of conspiring with demons. Besides there was never a chance for Niko to stay with the demons forever particularly considering they have different lifespans. Niko spreading her wings and leaving the nest is her decision and Maou is essentially immortal so even if it pains him, he is used to watching people move on before him