the fayth had lost control of Jecht and were being kept asleep by yu yevon
This.
If the fayth are kept dreaming by Yu Yevon, it's not like they could wake up and destroy Jecht to begin with, and even if they did, they would destroy Jecht, not Yu Yevon, who would proceed to float around and possess another aeon and create a new Sin.

That's what makes Yu Yevon so ironic, he's God and Sin at the same time. Without him, you can't summon the beings that you use to destroy him, thus that whole cycle thing that they talked about in the game.