So well-used is the so-called "Butterfly effect" in popular culture that I'm sure it doesn't need to be expounded upon, but still - is saving one girl in the past really enough to send the entire world into turmoil?
This certainly seems to be the case as Araragi and Shinobu return to the present after their successful jaunt to save Hachikuji from her fate - everything is wrecked and desolate,