The biggest reason to not meddle with the past and attempt to change it is the risk of getting trapped in that newly created branch of the timeline, in the same way that G'raha (or more specifically the Exarch immediately post-5.0) is trapped in ours. He's here willingly of course, but functionally he has no way of returning to the future he came from. If he fired up the time machine again and travelled forwards 200 years into the Source's future, he would arrive to a world completely different from the one he departed.

Travelling via Elidibus's portal might negate that risk, since it is tethered to a fixed point in our own timeline and not a permanent leap across time and space like the Crystal TARDIS, but it's still an unknown until we actually attempted it.

From a practical gameplay/storytelling perspective, the amount of things we would have to do to save the ancient world would boil down to "talking and politics, then repeat what we did in Endwalker to defeat Meteion in this timeline".

I also feel like it doesn't change things enough. Yes, you create a second better timeline for them, but you can't spare them the pain that they are destined to experience in this one.


On a side note, I think they should have leaned harder into the hopelessness of the Eighth Calamity situation, to make it more exceptional and justifiable as to why they didn't just carry on and try to fix things. There needed to be no recovery. No "endless age of war" – not enough people left for that. Just the last survivors scraping together and hoping they can complete this mad plan to give the past a second chance at life before the silent death of Black Rose claims them all.

That scenario would then create a better answer to this question: the other timeline was a literal dead end with no recovery; the only way to restore it was to wind back the clock and do it better. But the aftermath of the Sundering is recoverable and our present day is proof of it.