The universe was a perilous place. The countless nameless worlds which used to harbour life were an undeniable testament of that. We reached for the stars in a short window of opportunity we were granted and ultimately we prevailed. We saved ourselves, but in process our planet, our mother had to pay the price. After all the sterile oceans of our homeworld was an undeniable testament of that.
The Na'vi perceived us as terminally ill, insane race, an organism at war with itself and they were more or less right of course, but there was a grain of sanity to our madness. Ironically it was us who got to play the key role in preserving the balance of life, not their precious Eywa. After all the asteroid we prevented from obliterating Pandora was another undeniable testament still floating the depths of space.
We found our purpose in this universe and Earth's sacrifice wouldn't be in vain. The countless worlds we were about to save and spread the life to were going to serve as undeniable testament to that.