I kinda wonder what the scope of this show is going to be anyways. Like how many seasons are they planning on? Clearly at this point, they're not in danger of cancellation, so they probably are going to have to choose when to end. Personally, if the quality remained consistent, I could probably watch this show for a very long time.
But maybe there's a point at which they will be satisfied with this story? Like maybe Rick dying or something. I don't know what their ultimate goal is, but I'm guessing the ending is not going to be the end of zombies or anything happy like that.
I don't think that they have an endgame in mind. I know that Kirkman recently stated something to the effect that he'd love to keep this going for 25 seasons and the comics show no sign of ending.
Personally I think that the best sort of ending will be one where they focus on the relationship between Rick and Carl. Specifically, it should be about Rick's attempts to keep his son safe, help in grow up, teach him to survive and do all of that without becoming one of the living monsters that have become just as frighting as dead.
Assuming that they will never find a remedy to the Zombie Plague, the larger story should be about how humanity adapts to no longer being exclusively at the top of the food chain. What kind of world will Rick and the other surviors leave behind. Will humanity just revert to being savages or will we find a way to retain our better selves and create a future worth living for.
The question of why continue, why go on, and why bring kids into that situation keeps coming up. That's the question that should finally get answered.