[It honestly doesn't sound too different from what happened in Sornieth's Second Age. The machine that the people had built had exploded, wiping out all life. And in spite of that, life began anew.]

As I said before, mana is infinite. There's no way they can fully deplete it. And even if all life is destroyed, life will begin anew. I know this, because it has happened before in Sornieth.