I like hypotheticals for the sake of discussion, so spare me a moment if you can: Let's say Venat warned the people of Etheirys of Metion, who she is, what she became, so on. What would the people have Etheirys done? Personally, with what I have seen, I think they would have done about the same as they did for the Final Days. Construct a way to protect their star at all cost. Hermes would have been little help, due to grief or guilt of his creation, even if he explained what may have happened, he would have not found the will to aid them past that initial explanation based on how I see him as a character. And of course if Venat attempted to explain our whole time traveling thing, that would have opened up a whole new can of worms. With this information, I can see the Convocation working on a means to save their start over chasing after Metion to the edge of the universe. This addresses the immediate issue(The Final days), but not the problem itself(hopelessness), and considering tales akin to our Azem charging into a volcano were taken as very extreme and outlandish, I see no reason to believe they would actually put forth any efforts to stop Metion over just protecting their star assuming they could even defeat Metion in the first place.

While Etheirys might have been saved, there would have potentially existed some issues. First, Metion would have still existed, belting out the Song of Oblivion for all eternity that we know of. So even if Etheirys was shielded forever, her presences means everything else in the cosmos dies(very sad that). Second, we have no idea if Etheirys could even come up with a permanent solution to the Song of Oblivion, one that wouldn't deteriorate over time like waves against a rock. Third, if the answer to saving their planet was still Zodiark, I still have my doubts that it would have worked perfectly according to plan. Not to mention the effects of the all powerful primal "tugging" at the strings at their potential of being tempered. Lastly, there is still the issue of them becoming doomed like every other civilization Metion discovered, whether it do to similar effects like The Plenty, or maybe more internal conflicts involving Zodiark if that was used as a solution(I'd think that even with their knowledge, the "tugging" of Zodiark would still result in potential zealots, leading to infighting, and perhaps an all out civil war of Zodiark vs No-Zodiark like we were led to believe in ShB, or perhaps something worse).

Not to mention the main problem we were pointed to wasn't Metion, but the overall despair that brought the end to all those civilizations in one way or another. The best way forward, especially in the eyes of Venat, to prevent BOTH the Final Days and the potential of a "The Plenty" outcome, was to accept despair and find meaning in life accompanied by it. If Venat believed that the alerting of Metion would only bring about a temporary solution and detract the people of Etheirys from the main problem at hand, then it would make sense for her to NOT tell them the truth of the matter and instead have them focus on the root of the issue of that being existential dread. I only think this way due to the Elpis cut scene, where Venat is confronting a group of people on Ethirys, pleading for them to accept despair, accept loss, and move forward regardless. When the people chose not to tackle the main issue and instead chose a way to shield themselves from it, she most likely thought that they were inevitably doomed, either immediately or in the far future.

However, when our character regales our adventures with Venat, through our story we seemingly prove that we are a people who meet that criteria. A people who embrace despairs inevitability and push forward again, and again. So, with Venat having the knowledge she had, knowing that that the solution was to tackle the main issue of despair head on to prevent both destruction via Metion and to also circumvent the outcome of many other civilizations. Keeping Metions knowledge a secret was part of her own "test" to see if the current people of Ehteirys were capable of overcoming what stopped so many before them. So in the Elpis cut scene when Venat pleads for them to tackle the main issue at hand, and they basically say no, she knows that there exists in a post-sundered future people who would have answered yes, and thus elected to sunder the star. Knowing that there were people more than just our character, who would tackle despair head on, who wouldn't think twice about going out to the edge of the universe,(plus there is that whole "we are better at dynamis than you" thing), who were probably the best bet on a not doomed future.

I've been reading this thread for a while, so I know many of the counter arguments, concerns, and plot holes that exist. However this thought processes make sense to me in what they might have been attempting to portray that fits the narrative of the current story. Hopefully we get more information later on, through perhaps Pandemonium, that expands or details what we have a little better, before it is all dropped for something new.