The Ascian Culture and Society couldn't have defeated the Undending Despair.
I do agree that it's a really interesting situation where all sides are sort of both villainous and heroic not only depending on perspective, but also in that none of them KNEW their solution was actually the better one. However, we were only able to defeat Meteon BECAUSE The Warrior of Light and The Scions were born in the world full of suffering that Venat made, so I think that's why Emet Selch mentions that his plan wouldn't have worked, as he wanted you to become a rather mindless Light Warden, and the reason you and the scions were able to harness dynamis so well in order to win was BECAUSE you had a world of people to save. I think that's why Emet says his plan wouldn't have worked, as even if The Ascians have access to dynamis (we don't know to what extent as they barely knew about it), they did NOT have the tenacity and grit born from our brief lifespans of suffering described in Answers.
There's also the point that Venat did what she did, because she knew from our time travel that it would work to some extent. She probably also connected the dots and realized that it was a time loop, just as we could have at certain points. She heard our story and noticed all of the powers we overcame, all of the triumphs we shouldn't have had the strength to overcome (as almost every major villain ponders upon being defeated), and realizes that even over multiple millennium, with the core three restoring the memories of the rest of the 14 who could work on and ponder solutions, the Ascians STILL hadn't figured out Dynamis. She also knows that Aether and Dynamis don't interact, so winning with Aether would be quite impossible, which is a problem without The Source's creation as Ascians seem to be far too intertwined with aether (as Emet Selch seems to realize when he admits his plans wouldn't have worked). Sure, she may not have KNOWN that Ascians couldn't harness dynamis, but her people were sacrificing themselves at a rapid rate, so if she took the risk to keep the Ascians, they probably would've suffered the same fate as all the other stars, rather than sacrificing themselves to create an actual solution, which is what she forced them to do.
That's how I see it anyway. I also think Emet, in addition to many other plans, was gauging our strength to test if the people of the source were made whole enough, with the current number of rejoinings, to sacrifice to Zodiak in exchange for his people. Not that he was straight up planning on killing us and betraying us like that, but rather as an option, since that's what the Ascians were planning on doing until Venat intervened.
Hermes' Justification is WHY Venat can't just work with the Ascians
I was thinking Venat sort of jumped the gun, but now that I think about it, there was a lot of pressure to drastic action sooner than later. Even since the beginning with The Ultima Weapon, Hydaelyn exhausted all the strength she could to keep us alive, so had she waited ANY LONGER to do the Hydaelyn plan or something similar, then Hydaelyn wouldn't have been powerful enough to do the many life-saving, and therefore universe saving, things required to nurture The Warrior of Light into a strong enough weapon. MAYBE we could've won with Emet Selch, however she KNEW that going with the time loop would guarantee a force strong enough to defeat: 1v2 Igoram and Lahabrea (essentially in soul strength terms adding up it would be at most 1 Ascian worth of souls and aether), defeat Emet Selch WHILE ON THE VERGE OF DEATH, defeat Elidibus both as The Emissary holding the power of all of the souls of 75% of the Ascian race (or whatever power is left over from keeping the earth alive) and as Elidibus The Warrior of Light PRIMAL wielding the power OF THE PRAYERS of an army of Warrior of Lights, The Ultima Weapon whatever that means to her (I haven't done Endwalker patches or raids yet, so don't spoil Pandaemonium for me), and who knows what else she saw. It also doesn't seem to be an impulse reaction, since a lot of time, and a lot of sacrifices, took place between when she learns this and when she sunders everyone.
[2/3]... whoops
Venat saw the same flaw as Hermes, a flaw fundemental to Ascian identity
Had the 14 known, collectively they couldn't have justified or agreed on making the ultimate sacrifice of EVERYONE like she made, but had she not done that, The Ascians probably wouldn't have won considering all the pieces that line up for our Final Day fight (Zenos, fake echo, Ultima, Shinryu, all the present scions, including time-traveled apocalypse survivor G'raha and reformed Vengence addict Estinien, Omega, Alpha ... and so forth due to Zenos & Shinryu just about everything significant in history is tied into this moment), and it took all of our friend's VARIETY of experiences and DIFFERENCES that allowed us to Close in the Distance and win. We won because this world of suffering that Venat made gave us the variety of sufferings overcome and variety of strengths and experiences to overcome fallen worlds full of suffered souls and unending despair. Venats choice works BECAUSE Hermes wasn't just maliciously villainous, he was broken by the faults in Ascian society and culture, so if Venat HAD gambled into a new unknown timeline, then that wouldn't fix the fault Hermes broke over where The Ascians missed that the point of any individual life IS to find their own reason for living. With Emet's plan, they'd end up just like the other worlds who achieve whatever universal cultural goals their societies decide is most important, rather than ending up with a diverse world full of different goals and dreams. Just as Emet States, The Asians were more alike to one another than different (I couldn't find the exact quote, and have NO recollection of when he said it), and as The Traveler, Venat understood that BEFORE Hermes or the final days, so she had likely been thinking about it for a long time, and the time travel loop knowledge only confirmed the idea and the final days, slowly losing everyone, pushed her to realize that the fault was WITH The Ascians as they were, and if this UNIVERSE ENDING threat was to be vanquished, she had to act before it was too late.
WOW I REALLY wish I could find a writing job I'm interested in, as I just ADHD zoned out and typed this out for WAY too long.