if they could accurately visualize that hypothetical world in which there was no rationality and they themselves have become irrational?

I just attempted to visualize such a world, and my mind ran into a brick wall. I can easily imagine a world in which I am not perfectly rational (and in fact am barely rational at all), and that world looks a lot like this world. But I can't imagine a world in which rationality doesn't exist, except as a world in which no decision-making entities exist. Because in any world in which there exist better and worse options and an entity that can model those options and choose between them with better than random chance, there exists a certain amount of rationality.