I don't really think Necron is completely bullshit... at least not the way I saw it...If Necron wasn't there for the party to beat, the world would be over the moment Kuja destroyed the crystal...
There were two things that had to happen so the world would end:
1 destroy the crystal, 2: let the "darkness of the world" do the job.
By stopping the second they let the world continue, even though the first thing happened =/
Had the party managed to defeat Kuja before he destroyed the crystal, they would still need to handle the destruction of the portal to memoria and the Iifa tree, and if they stayed there to destroy it the whole party would die, since I don't really think anyone there could destroy it alone, like Kuja did.
I say this because they told us that Kuja would die when Zidane managed to be stronger than him, and that never happened in the game, Kuja is the only final boss that never got beaten by the heroes, and him being alive made it possible for the party to come back home safely(well, most of it), the moment Zidane surpassed that strenght Kuja would die(and yeah, I do know this might mean I think he was alive at the end of the game... or that the fight against necron made Zidane that stronger, I like to believe the former because if his lifespan is that short, then you'd have to believe zidane will die at 24 too, and that's kinda sad D: )
At least that's how I see things so I don't think that game got screwed up, if we beat Kuja someone would have a tragic death so the world wouldn't be covered in mist forever, if there wasn't a chance to beat Necron the world would be over. Either way, we'd never see Vivi's adorable sons, or zidane coming back or any happy ending whatsoever.
The fear of death made Kuja destroy the world, the fear of death made Terra absorve Gaia so their inhabitants would live... I you look that way, that was the game's villain all the time: that's what Necron is, they just didn't explain/translated it so well, hell, if Kuja didn't know 'bout zidane and how he wold be over, i don't think he would've made all that mess, he was just Garland's puppet after all, thats why zidane came back in the end =/
As to why they even got a chance to beat that force instead of just ending the world, well, that's the part where I have to say "well, it's a game, ya'know" D: