That'd be a pretty disappointing ending imo. It's something that's been done to death in so many stories. Good triumphs. Heroes, some from almost humble beginnings, others redeeming their families prevail. Super mega happy ending. Sing a song. Dance a dance. Cut to black.
That's kinda why part of me wants the Night King to win. Doesn't kill all the characters and population. Ushers in the long night again. Sets up a sequel series with the remaining characters. Like After MASH but actually good and watchable.
A prequel series has already been said to be in the works. That's also kind of disappointing. Mainly because prequels are generally bad with numerous plot holes. Hope that's not the case with the GoT series.