Except Hermes would have to be brought into the fold if all the Convocation were to know - which would work against her because now Hermes could turn against them and they would not have someone who can replace him in the Convocation who also excels in research of outer space & aetherology for future problems (and his expertise was what led them to discover the stagnation of the aether currents & figure out the strategy to forestall the Final Days in the first place). If they let him know before Meteion unleashed the Final Days, he would've turned against them because Venat was supposed to have lost her memories when during their journey in Ktisis Hyperboreia. That would be a worse outcome because now they can't forestall The Final Days at all and the Final Days would've just ended the Ancients then and there after affecting their ability to manipulate aether and creating more blasphemies.
So they had to overcome the Final Days without Venat's direct intervention first for Hermes to accept and judge mankind to be able to pass Meteion's judgement... Except that never happened because the ancients failed and resorted to summoning Zodiark to return to their paradise and wouldn't listen otherwise when Venat tried to get them to accept that immutable truth that suffering exists and their world isn't perfect; even if they go back, pain and despair will still remain (the events of the Endwalker raids prove this to be true too).
This was why the events that led up to our time was pre-destined to happen and Argos ended up being friendly to us. Venat couldn't do anything by the circumstances involving Hermes' resolve and judgement without clueing him in that she remembers and will make the judgement unfair. She could only try to persuade the people. Her attempt was basically always going to fail in our timeline since he was integral to both causing and stopping The Final Days.