That's always been on of the ponderous aspects of GOT that I've wondered on.The irony is that even if the Wall doesn't fall and the Night King never moves south, half of Westeros is gonna starve to death anyway because they're going into winter with deeply depleted food reserves. And of those kingdoms that have been relatively untouched by war, the Reach is getting pillaged by Euron and the Stormlands are getting conquered by Aegon. That basically leaves Dorne as the only nation with an intact food supply going into winter (the Vale is struggling with the mountain clans and the Westerlands probably used up a lot of food during the war of the five kings). By the time Dany gets her shit together and rolls into Westeros there's going to be nothing left to conquer. Which is probably a good thing for her, because, aside from her dragons, her army is pathetically unequipped to mount a winter siege against a fortified castle.
As the war drags out, the material and troops for the political infighting will have to reach an end point as well. Stannis was defeated by the Northern climate, more than by fighting Ramsay Bolton.
The North seemingly has been cleared out of available troops by five years of war in the south and essentially civil war at home, even with the wildling refugees, there isn't enough material left to actually equip a strong formal army.
The Riverland's has been cleared out by the insurrection as well, and there isn't a single house outside of the Martell's or Dorne that hasn't suffered larger scale casualties, from the civil war. Even the Lannister have been running the kingdom on borrowed time and money.
The real equalizer will probably be Daenyris with her slave army, dragons, and seemingly endless hordes of Dothraki, (or that fact that everything seemingly just falls into her lap because muh breaker of chains mother of dragons, she really is the most boring character in the books) but again the book never emphasized whether this will be a war fought in the south against Cersei or if she'd come to the north.