For everyone complaining that the Army of the Dead was built up as the big bad only to get defeated in one episode, remember that:
- They'd been killing the wildlings (and a few crows) for years
- They kicked the NW's ass at the Fist of the First Men
- They kicked the NW and wildling's asses at Hard Home
- They killed a dragon
- The destroyed the wall and Eastwatch
- the ran straight over Last Hearth
- They pretty much wiped out all the Dothraki and Unsullied
Just seemed painfully evident that there would not, could not, be another major engagement with the NK. He either dies or you lose. No regroup and fight another day.
Further, and I know this is disappointing to many, but it also seems clear the NK and WW's really didn't possess any level of complexity. TCOTF created him for one purpose, kill the humans, and that's all his purpose was.
And if there was ever going to be anything more to that part of the story we'll probably never know because the last time GRRM wrote anything significant about the WW's was book 2 over 20 years ago.