I think it's kind of an unfair assertion to make, given the nature of time travel. We don't know Venat didn't do everything in her power to prevent the Rejoinings, it just so happens that our timeline exists as a consequence of the OG Final Days / Sundering / seven Rejoinings so we can't observe timelines where any of those didn't happen. You can propose staying in the past but Venat tells you not to because it could get dangerous (temporal paradox, ayy), but point is nothing we did or saw on Elpis could meaningfully alter the (relative) past. Elidibus warned us as much.

Hydaelyn admits she's responsible for the Sundering and that it was an abhorrent thing to do, not the least because it's caused innumerable avoidable deaths, but it was necessary for humanity to move forward and rise to Hermes' challenge (thus defeating Meteion and saving the star). It's a far cry from the Ascians imploding the fractured worlds and killing their sundered brethren en masse for the sake of their regressive nostalgia.

There can be a difference between what is right and what is necessary. Moral quandaries, I know.