I think a big factor in Elpis' ending is that Hermes is, essentially, in an existential crisis. He's sent the Meteia out in hopes that they will be able to find some nobler truth, some more meaningful philosophy, than the perfect world the Ascians envision.
The 'truth' meteion finds will absolutely crush him, and she knows it, that's why she flees and tries to suppress it. If you seek hope among the stars, a reason to live, and find every single civilisation has either died or wishes for death, it's not at all surprising he'd go mad from the revelation.
As for Venat, a big problem is that very few if any people know what Dynamis/Akasa is, and even less would know Meteion is made of Dynamis. You can't really discuss Meteion's fall and the risk of the final days without including hermes, and if you do that, you risk a relapse of Hermes into his madness. Emet struggled to believe us with our direct testimony, and watching our actions from the moment we arrived. The only reason he remotely accepts we're from the future is that Venat only meets us later. If she arrives with us, that one piece of evidence is going to fall apart. There's another matter you don't quite have the full picture yet, spoilers for just before the last zone:
Another reason for the split is to make the people of Etheirys emotionally stronger and better at dealing with hardship, as well as being able to wield dynamis to a degree. Venat still acknowledges that she is basically the root cause for nearly all the suffering in the world, but without her, they'd all just blindly worship Zodiark, and sacrifice other lives to rebirth their friends from him (which is entirely in keeping with their actions in Elpis).
@Zeke Von Genbu is wrong on the time travel aspect, as revealed in some of the tales from the shadows. The post-eighth-calamity world still exists as it's own timeline, but creating our timeline didn't save their one, and as far as we know, G'Raha can't return to the world he came from. By extension, if we change the timeline, such that the Ascians are saved, we won't be able to save our own.