As I'd pointed out in another thread, they have actually established in the external lore that Aerith was the one who managed to use the lifestream to stop Meteor (or at least they heavily indicate that). I'm not sure what they'd do with stuff like that if she lives.

I'm sticking with my theory that she'll survive in the alternate timeline with Zack instead. Given that they were the original couple, and Cloud should be with Tifa.