"My hate... never perishes. It is born anew in a cycle with no end! I will rise again! Those like you... Those who share the blood of the goddess and the spirit of the hero... They are eternally bound to this curse. An incarnation of my hatred shall ever follow your kind, dooming them to wander a blood-soaked sea of darkness for all time!" - Demise
I don’t view these things as eternal forces of nature that were created in that brief moment simply by a curse Demise set in motion with his dying breath. With all the influences of eastern culture in this game, Buddhism in particular, the theology of reincarnation in the game is more than likely closer to the philosophy in Buddhism itself, rather than being some new version fitted into the game. In Buddhist theology, reincarnation is a naturally occurring cycle that has simply existed from the beginning of time, and which will always continue existing as a force within temporal cosmology, thus, nothing ever truly stays dead, everything is reborn anew. I think there is enough in game evidence to say the cycles of reincarnation at work in Skyward Sword are more akin to this theology.
During the battle with Demise, Fi gives some very interesting info.
"This eternal being has conquered time itself. It is the source of all monsters. According to tales passed down through generations, it appears differently in each epoch and to each person who lays eyes on it." - Fi
This quote might confirm that the ancestors of the Skyloftians, and definitely Hylia knew that Demise, as a force of evil, cannot be completely destroyed. Demise knew this about himself as well it seems. The hate curse itself doesn’t come off to me like a last minute thing, it’s more like Demise is pointing out something that Link is unaware of as a sort of last laugh. Consider the way it’s worded. He begins by establishing that his hate never perishes, and it will continuously be reborn, and this is before he even goes on to set his “curse”. The Japanese version of the dialogue is slightly different, and stresses this isn’t some random force coming into play suddenly.
"My hatred... The curse of the Demon Tribe... They shall continously go on reincarnating until the end of all times.
Do not forget it! I shall repeat it!!
You people shall... You people who possess the blood of the Goddess and the soul of hero shall... forever be unable to escape from this curse!
This hatred and grudge... Its evolution shall forever painfully wander across this blood-stained "Dark Sea" along with you lowlifes forever!!” - Japanese Translation of Demise’s hate curse
The first section establishes it as a force already in play within the “Demon Tribe”, which as a whole is the army that Demise led against Hylia, mentioned in the game’s intro scene. So this at least gives some support to my interpretation of the curse.
To clarify, I’m not saying the curse is metaphorical, I view it as a literal curse,or cycle. Afterall, the whole point of the curse is to give an explanation for Ganondorf, and possibly other villians in the series as well. In Shinto, everything is part of nature and shares the interrelated complexity that is Kami, or the divine essence. It’s also noteworthy that a curse in this sense is defined as “invoking a supernatural power”. It’s not a power that has been created on the spot, it’s a pre-established power that is being tapped into to torment a person, or group. With all this considered, it makes more sense that the curse existed long before Demise mentioned it, or at least the groundwork for the curse was already set, and it just took Demise invoking it to really set it in motion, maybe.