I honestly think would have at least had a chance if all of Spira worked together. Maybe...The best weapons are made during war, after all. But like Sir Taffy said, Sin always kept that from happening, so nothing powerful enough would ever get made as things were. Besides, humans (or other races as Spira has them) have never broken their self-made factions to fight as a unit no matter what the threat is. If aliens attacked earth and had superior weapons, humankind would not bond together to fight them as one, we would each fight as separate countries and we would all die. I suppose they got close singing the hymn altogether at the end, but look at what Yuna and company had to go through just to get that done.

Plus, to the player, Sin isn't very tough. I mean, Tidus is able to whack away at him for 99,999 damage. Gameplay-wise, he's not very threatening. But within the world and within the plot, he's supposed to be an almighty badass. Enemies get special powers in cutscene-land, so I just doubt many machica in any number would defeat him there.