What had the Lady Jessica to sustain her in her time of trial? Think you carefully on this Bene Gesserit proverb and perhaps you will see: "Any road followed precisely to its end leads precisely nowhere. Climb the mountain just a little bit to test that it's a mountain. From the top of the mountain, you cannot see the mountain."
It is said that the Duke Leto blinded himself to the perils of Arrakis, that he walked heedlessly into the pit. Would it not be more likely to suggest he had lived so long in the presence of extreme danger he misjudged a change in its intensity? Or is it possible he deliberately sacrificed himself that his son might find a better life? All evidence indicates the Duke was a man not easily hoodwinked.