Also afterwards she says this:

"And this hinges, I think, on how we confront the all-consuming despair that accompanies a senseless and seemingly inevitable end. Bewildered and divided, we would perish like the peoples of those celestial ruins. We could not hope to survive the Final Days, much less take the battle to Meteion at her nest. We must find a way to defeat despair. To unite and prepare as many as possible for the struggle ahead."

So yeah she did told people about the situation before the first Final Days happened. But realistically the Ancients would'nt have been strong enough in both heart and mind in their current state (as well as their lack of controlling dynamis) to be able to survive Ultima Thule and confront Meteion. Even if they knew how to create entelechies, it would've been seriously flawed due to the proper lack of understanding suffering and using that as a strength rather than a weakness like Meteion chose. Which is why despite everything, the sundering had to happen. Not only for the people of Etheryis to be able to control and use dynamis but for their own development to eventually find the answer and strength needed to combat the song of Oblivion.