If the Knight King and his dead zombies win; there will be no future. They will simply go back into hibernation and go dormant. Surviving humans should have future because the zombies can’t cross water without a ship. Of course all it would take is the Knight King and a few dead zombies to cross to the Iron Islands on one ship then every new dead person would rise as a Knight King zombie until all living humans were eventually eliminated.
Just thinking out loud...
It’s a cycle! Who ultimately lives or dies each cycle is determined by the season. Summer comes the Snow Zombies, etc. die, melt, catch hay fever whatever. Their remnant retreat to that world’s north pole. (Ummm they don’t seem to have a south pole.) Humans re-expand have their spring\summer civilization, winter comes it collapse. The cycle repeats.
Also my theory allows the two contesting civilizations & environmental conditions to be mirror images of each other.
Thus continuing the liberal mantra no good, no bad just a sort of yin/yang. Martin is very much the modern day liberal. He is no JRR Tolkien!
They could choose to go after the islands. Float the sleeping dead on ice bergs, ships and debris to see if one can reach an island. Then wake up and start infecting new people.
Fly infected dead ravens to the islands, convert other animals, then upgrade to humans.
Knight King does not need a ship. He has a flying dragon.
He does not need zombies. He re-animates the dead by himself. All he needs to do is land at any island, kill a few, re-animate them, and the process starts over.