In my opinion, maybe if they hadn't rushed. That tower thing got real close, but wasn't enough, and even if it broke through, they still have to get through the layers of Sin. Maybe if they got to salvage 2 or 3 more then, they probably could've at least done some damage. The problem is Sin would've killed them before they got the chance. IMO it would've been possible to defeat Sin -if- the people of Spira didn't believe so strongly in Yevon. During the calm they should've been gathering machina, and having a Summoner getting ready to bring another calm, so that way they have more time.