Gestahl didn't seem to notice things like Kefka standing between the statues to avoid magic attacks and didn't seem like he was going to move the statues out of alignment, so I honestly think that if Kefka hadn't done anything then the party would have either killed them both and ended it there, or they would have been killed by Gestahl and Kefka's superior magical abilities and the rest of the group would have been hunted down and slaughtered if they weren't all killed on the airship.

Either way, I'm not sure there's be much of a second half of the game. Which makes sense since the floating continent did feel like a bit of a climax in its own right.