It's always funny how facts like that only come up when Whedon's getting criticized, but not when he's being praised. It wasn't a system. It was a fact of the world. Without that system, there would be no world. The only possible variant would be one based on a lottery or other voluntary means of picking the sacrificial victims, but obviously that wasn't an option else any number of people within the organization would have willingly gave up their own lives in place of the chosen victims. It also means that no one, anywhere around the world in any country or any religion, was using such a method since only ONE group of sacrifices was necessary. As the movie made abundantly clear. What's more worrying is what we saw of the Japanese method. Apparently, an entire class of preteen schoolgirls would have appeased these gods in place of the "rules" required of the victims we saw in the United States. Or, amongst those preteen girls, was there a whore, a virgin (meaning all the others have been 'touched' at least once), and a stoner amongst them, and all those other poor girls were just being killed so the Japanese works could wank one off or something? Keep in mind that, again, only one nation around the world had to succeed at the sacrifice, which logically means it was to the same group of gods. So, either they just made up random rules to fuck with humanity all around the world and any random killing would work despite the specific archetypes/statues focused on in the actual movie, or Whedon just tacked the "twist ending" on and hoped no one would notice.