How does "unmaking" happen? What power does Buffy have to "unmake" the Slayer line? Fray shows that Buffy does not unmake the line, because she (Fray) was the first to be called in centuries because the monsters returned. Using this information, we can amend the verb "unmake" to "postpone," and "assume" that Buffy et al. kill off all the vampires.

With the exception of Angel and Spike, of course;)