Ash and his friends weren't fading because Arceus was dying, per se.
- Pokémon Legends: Arceus alludes to Arceus being a mere fragment of a much greater, far more eldritch Entity; due to the scale of said entity, electricity and that metallic water would not have killed Arceus's true form. Rather, the reason why everyone in the future was fading is because Arceus, pissed that said piece of himself died, would have unleashed the raw power of his true form onto the universe as an act of vengeance-which would likely have wiped that entire timeline, and many other universes, from existence. So Ash and the others were fading because their entire universe was moments from annihilation, and there would be no way for them to exist.
