Here is a thought I just had (and decided to register to share
). One thing different about Hyrule in Skyward Sword from almost any other Zelda is its source of evil. It is clearly stated all the evil comes from demons, which Demise is king over. The mortal races are all good, in fact they are 100% completely and totally good. You don't once hear or see a Hylian, Goron, Mogma, etc really doing anything evil at all (the robots are rather rude, but not evil of course lol). The mortal races are never described as overally greedy, ambitious, or evil, not even in the back story. Compare that to the chronologically later Zeldas games and you get plenty of that. You get the whole Gerudo race, backstories of civil wars, whole armies of men in LttP fighting over the location of the Golden Land. None of this strife in Skyward Sword's time though. In fact I always got a naive, young vibe from the peoples in the game, everything felt very primordial in this time. Even the ending itself, with the Triforce just kind of chilling out in the open like that.
Anyway the point is what if Demise's curse was like Pandora's box in a way. The curse was to infect evil into the moral world, into mortal hearts and minds, which eventually manifests itself into the ultimate personification of MORTAL evil, Ganondorf. Maybe it was this curse that corrupted the mortal world, led to the need rehide the Triforce away from Mortal hands (the only beings we have now learned who can actually use it, a fact Demise must not of known).
In this way Demise is and isn't Ganondorf, he isn't directly Ganondorf in anyway, but Ganondorf is the most direct personification of evil in the later ages and is a mortal instead of a demon/god/big ugly fire guy.
Just a thought.