could humanity have survived?

[Roogle]

I think that it is necessary to consider that each incarnation of Sin has the persona and psyche of a Final Aeon inside of it. This certainly limits the amount of destruction inflicted by the monstrocity, and it furthers the reasoning of Seymour wanting to be a Final Aeon, as he does not want to keep Sin at bay.

[cally777]

Sin needed to be a being tortured between commands of Yu-Yevon and the love of its own people, its own loved ones. The whole journey of the summoner was meant to create strong emotional bonds, and to give them a fatalistic view of Spira. Sin, Yevon, Aeons, all of these things are apart of psychological warfare constructed within the mind of a being who is essentially immortal.

A very interesting comment because it seems you are implying that not only was the pilgrimage designed to prepare a Summoner to defeat Sin (temporarily), it was also to psychologically prepare the Final Aeon (typically a Guardian) to become Sin. A being torn between destructive impulses and protective ones. This is certainly one explanation of Sin's dual nature, and why it didn't destroy everything.