You’ve won.
Or
You’ve lost.
But the game won’t end.
The ultimate outcome is known. The rest is just theater.
Boring theater at that.
This is the ultimate fate of so many 4X games. You know you’re going to win (or lose) eventually but you still feel like you have to go through the motions of slogging it out.
For Beta 2, the AI will just surrender when it’s hopeless. But humans have the choice to fight on. Because there’s always hope.
In the game I’m playing right now, the AI has me down to my last city. But he can’t take that city.

They have me surrounded. But they can’t take the city. I am not sure they can.

And I can’t take them.
I’m down to my last city.
They’ve destroyed all my outposts. My shards are gone. My metal is gone. I have just this one city.
Different Ideas
Laying siege is how this sort of thing would historically work. Have the player lay siege and it’ll slowly wear me down. This fails because my army in the city is easily powerful enough to wipe out any individual army that would try to lay siege. Plus, it’s a pretty significant game mechanics to add, test, and balance this late in the game.
The idea I’m playing with tonight involves magic. If you can’t take the city by force, make it miserable to live there through magic until the city finally destroys itself if not countered (which I wouldn’t be able to since I have no mana).