For sure. It really made you consider what kind of person you would be in such a world. One who would, along with the loss of civilisation, lose their mind? Or one who somehow adapt in someway, even if that came long with coping mechanisms? I also really liked how the concept of time breaking down as Ish neared his death: "Time in its old sense of appointments to be kept and things to be done - all that had long since ceased to exist, both because the way of life had changed and because he himself was so old as to be almost out of life. In certain ways, he had already, as it seemed, passed from time to eternity." Thinking back, one element I would've liked to have seen explored more deeply is interaction with other tribes. I know they sent Dick & Bob on that expedition, but the details that emerged from it weren't too extensive. It was just a summary of what they'd encountered. Though I guess the book would've had to have been notably longer to fit it in, which isn't necessarily desirable. It certainly feels like a complete story as it already is.
I let the idea of this roll about in my mind over the last few days and I think you're right. Like you said, they do integrate with a nearby tribe at one point. And not that much is said about it because in reality the union of two peaceful tribes would likely be uneventful. In the event of there being a clash with something more threatening, you got a taste of that with the introduction of Charlie. It allowed for a perspective of how they would handle hostile outsiders without the need to divert the narrative to inter-tribe politics etc.