The "eighth and final" claim was from Fanfest (IIRC) rather than within the story, and people simply assumed this meant Zodiark could/would be revived after that. Even then it could be interpreted in other ways, and Shadowbringers makes that interpretation seem very unlikely.

I suspect in the bad timeline the Source may be damaged beyond repair - if Black Rose is still slowly spreading and poisoning / halting aether, civilisation may never recover and life may not survive long-term. The Ascians may have broken their own system and another rejoining is not possible. The eighth Calamity was the last, because they won't be able to set up another.

As for whether the other timeline still exists...? It's somewhat of a philosophical question when we have no way of checking. But if we assume the timeline is stable now, G'raha Tia still exists, therefore the timeline that formed his past *should* still exist.

The way it makes the most sense to me, especially when trying to resolve it against Alexander's "stable time loop" story format, is that this isn't a large-scale "multiple timelines" story where all sorts of things spawn off alternate timelines where things did/didn't happen. Rather, a unique set of circumstances have allowed them to create this one split in the timeline by creating a paradox that can't resolve itself into a single neat timeline. The 'old' timeline has to remain in existence because things from it (G'raha, the tower, information about events that happened there) cannot be erased without also preventing the changes that erased them. So both timelines stay open, though there may not be much of a future for people in the other timeline where it seems the world is dying. they just know that there's also another world out there where - if their plan succeeds - things have gone better.