The leaders of Westeros could make peace and then commit to staying forever vigilant against the undead threat. The show would end with the castles along The Wall getting rebuilt and people from every side on top, looking north toward a threat that can never be beaten, only contained.
That sounds like a deeply unentertaining ending, but remember that the actual show did end with all the surviving leaders meeting peacefully and deciding the fate of the land. This ending to the walker story would have to come after some battle over the throne—a battle that, afterward, all survivors agree wasted resources and distracted from the real issue.
HBO
because they forgot neither still served a purpose and both had been destroyed
Or, better than that, they could have ended with a big fight with the walkers after all. Except, the walkers would win this fight, easily. Picture the carnage we got when that dragon burned King’s Landing, except we’ll feel the actual horror the writers wanted us to feel, instead of just muttering to ourselves, “Why are they doing that? That doesn’t make any sense.”
Some named characters would survive. They’d take to the sea, because the entire continent is lost to the dead and winter. The iron throne? It’s the Night King’s now. Or, walkers rip the throne apart (which means something in this ending, unlike the dragon destroying the throne in the finale we got, because here, no more kingdom remains to rule over).
Cracked readers can probably think of 17 endings better than either of those. But all these better endings must have that undead swarm really be a bigger threat than any other army. In the show we got, they turned out to be less of a threat than any other army, because you could disintegrate them all simply by killing one keystone guy, and everyone knew it. Dammit, generals of Westeros. A week before the big battle, you could have just hired one of those magic assassins to kill the Night King, and that would have ended everything.