An asspull? Pah! Preposterous!!!
Scenario1:
Healing god turns into a fetus and dies. Saike uses lake Gokyo again and undoes his death. Logical happy end.
Scenario2:
Healing god saves Saike. Saike oversleeps and misses the window for going back in time to save the healing god. I feel cheated.
Scenario3:
Saike just forgets about his oracle and lets the healing god remain dead. I question his character and feel cheated. Really, really, cheated!
Scenario4:
Mangaka didn't want to draw / make us watch the beloved healing god, as a reward for his heroic deed, turn into a fetus and die horribly, even if it's only temporary...
So his oracle just shuts down instead. (Or maybe the mangaka was really into doing that, but his editor didn't let him)
I don't think there was any way of having the healing god die for good without it being an even bigger asspull in some way and at least this asspull forgoes having someone revert into a fucking fetus and die.
edit: And there is still Kuroda who could possibly prevent the healing god's death by sealing his oracle, as long as the de-aging isn't instant