Ive only played the ruthless route once. Wasn't that a fail safe elixir for tyrant rulers of the realm?
The only immortal knowledge that I have is things like Dracula, Lord of the Rings, Twilight, True Blood, Vamp. Diaries. Etc. So I'm inclined to believe you on that, but that doesn't mean it can't be for MC and Asterion. I think peace in the realm is very achievable (as long as the old gods have actually died out other than Hades) and as long as we go on this quest without some casualties or maiming involved. Neither the MC nor Asterion truly desire a tyrannical rule of the realm (barring the Ruthless Route),
(Edit) This is still speculation and theory crafting but, I feel Hades out of all the Olympian's has the best chance of never fading away, due to his subjects actually being there and still giving him reverence in his realm. His power seem to differ from the rest of the hierarchy on Olympus. Maybe there is loop hole or even a surprise communication from the benevolent under lord instructing our cast further if Hermes doesn't survive the ordeal of waiting for the Ichor.