You are aware that, by altering the timeline, they were essentially killing all entities that have been born past the incursion point?
They did this so they wouldn't lose anyone, they stood to gain a lot if it works.
This isn't "selflessness" at all. In fact, going back and "fixing" the future because it turned out in a way you dislike is on of the oldest topics of Sci-Fi and generally considered unethical.
The real kicker of the entire shabang is: with their knowledge, sophistication and sheer limitless power, why didn't the Ascians
a) have the same idea.
b) see what was going on under their noses and put a stop to it.