Seht was not an idiot as you all seem to imply.
Idiocy is different than accepting the inevitability of death. The heart and his power source was gone, his power was waning, he was eventually going to die one way or another. Perhaps it was a realization that he lived a lot longer than he natually should have, and resigned himself to his fate. The same as Vehk resigned to his fate of no longer being the god he once was (as opposed to finding a way to get back to the heart and restore his power). Ayem was the one that wouldn't accept her fate and went crazy, and nothing Seht could've said or done would change that.
More importantly, it seems to fit the trinity thing they have going on...
Sotha Sil was the mage, Almalexia was the warrior, Vivec was the thief (ie, warrior-mage).
Sotha Sil was he, Almalexia was she, Vivec was he-she.
Sotha Sil accepted death, Almalexia accepted nothing, Vivec accepted loss of divinity.
That's how I'd see it, anyway.