Debatable.
Oh good, I love debates.
If Seswatha is dead, then the Anasurimbor's never left Earwa, and the whole prophecy is nonsense. The Dunyain, and thus the Anasurimbor's, survived in the north, but were removed from the scene, allowing for a "return" even though they never left. I think this shows enough ambiguity to say that while Seswatha's body may have perished, he remains alive in the minds/souls of the Mandate and Swayali.
If an Anasurimbor can return from where he never left, Seswatha can be alive even though he is dead.
We know that the first half of the prophecy is true. The World, which I take to mean humanity, is in peril. They are all at risk of being obliterated thanks to Kellhus and the Inchoroi. The Great Ordeal would never have happened without Moenghus and Kellhus, so one of them was indeed the Harbinger.
Again, because the first half of the prophecy is shown to be true, so too should we consider the second half to be true. If all those connected to Seswatha die, the world will end. Without any Gnostic school(wo)men, there is nothing to oppose the Consult, the world is lost.