Эльф
Все творимые Вечностью изменения были волной, всплеск, сильное влияние, а потом затухают, и им приходилось вмешиваться еще и еще, бесконечно. Скрытые столетия не исчезли бы, уничтожив Вечность, но изменилась бы их Реальность, защита им была нужна временно, чтобы Вечность не могла влезть напрямую в их столетия. Цивилизация выродилась, потому что стало не к чему стремиться. Бытовое есть, а дальше ноль.
Цитата про вымирание из оригинала, не было ничего про военные конфликты. Подозреваю, к моменту вылета в космос, у человечества уже не было понятия войн.
"And once they could reach the stars, they did so and left the Earth. Some of us have guessed that."
"Then some of you have guessed wrong. Man tried to leave Earth. Unfortunately, however, we are not alone in the Galaxy. There are other stars with other planets, you know. There are even other intelligences. None, in this Galaxy at least, are as ancient as mankind, but in the 125,000 Centuries man remained on Earth, younger minds caught up and passed us, developed the interstellar drive, and colonized the Galaxy.
"When we moved out into space, the signs were up. _Occupied! No Trespassing! Clear Out!_ Mankind drew back its exploratory feelers, remained at home. But now he knew Earth for what it was: a prison surrounded by an infinity of freedom... And mankind died out!"
Harlan said, "Just died out. Nonsense."
"They didn't just die out. It took thousands of Centuries. There were ups and downs but, on the whole, there was a loss of purpose, a sense of futility, a feeling of hopelessness that could not be overcome. Eventually there was one last decline of the birth rate and finally, extinction. Your Eternity did that."