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, with not a human in sight (not that this is all that uncommon for the town in this show, mind you) and everything brought to a grinding halt. With no opportunity for further time travel, and with the nightfall bringing with it "zombies" en masse, it seems as if there is no escape for our surviving pair.
The real horror of the situation, however, is that Shinobu might just be to blame for the entire thing. As she relates her story of the day she went missing and her desire to annihilate the world if Araragi weren't to find her, the pieces begin to click into place - Koyomi only found Shinobu thanks to a tip-off from Hachikuji, and without that knowledge it seems that Shinobu started a vicious circle that turned all of humanity into vampires, before dying herself to effectively make those vampires masterless zombies. With seemingly no way to fix this disastrous turn of events, Koyomi and Shinobu resign themselves to simply existing, and calling out via fireworks in the hope of at least a single other survivor.
If nothing else, this is certainly a very different story arc than normal Monogatari fare - here is a series that tends to keep its focus very tight on a singular character, yet suddenly here we are with the entirety of humanity in peril (to put it mildly). Whether this concept will work for this show remains to be seen - I still haven't entirely "bought in" to this story arc for some reason, but I will give it some kudos for having sufficiently smart reasoning to back up its existence. I'm more than a little curious to see where the story goes from here though, so hopefully it can deliver with the kind of panache that the Monogatari franchise has virtually made its calling card.