First order of business: Giles begins his explanation by telling us "Nature isn't wasteful. It doesn't put anything in the ecosystem that it doesn't need." This is not true. Nature puts useless items into the ecosystem all the time: dead-end mutations, biological by-products of necessary traits, boy bands, etc. The reason that the balance stays intact in spite of these items is that they depend on the support of the rest of the ecosystem, and if they aren't serving a purpose, they can't hold their own against competitors and they fade out.
And indeed, the next line is "If one side feeds too much, smaller animals lose their food and the entire ecosystem collapses." True, of course, but it doesn't follow from the idea of nothing wasteful going into the system. Nature has no safeguard against the eventuality of a mass extinction; extinction is the safeguard. Remember when the dinosaurs died out? Well, no, you weren't there. I wasn't either. Some fuzzy lil' shrews were there, though, and without needing to worry about dinosaurs all the time, they got bigger and smarter and turned into us. For all intents and purposes, the ecosystem had collapsed. Evolution went on, full speed ahead.
So, what we're meant to believe is that Slayers were introduced to the supernatural world to balance vampires and keep them from feeding too much. Why would this be? If the illustration of the natural ecosystem meant anything at all, why shouldn't we assume that the supernatural ecosystem would just allow vampires to keep feeding until their species suffered the inevitable result of overpopulation?
The only answer we're getting is that the Universe has a plan. It's 'smart.' And until we got to that concept I believed everything Book Guy Giles told me, but here I'm just not picking up what he's throwing down. If the Universe has a plan, it has nothing to do with any form of evolution any of us have yet encountered. A common conceit of human thought is that we, the most complex and powerful creatures in our world, are a final product, the end result of billions of years of evolution's craftsmanship. In actuality, Nature has no need or ability to aim for complexity; the simplest option is always the default. Organisms evolved from bacteria because bacteria had been so successful that there was no more room in its ecological slot for more of it. Exoskeletony-type creatures crawled out of the water because the exoskeleton party had grown too big to fit in the water and the land was at that point untapped as a resource. Cavemen invented a hand-axe because the other predators didn't have hand-axes. Bacteria remain the most successful form of life on earth.
I'm willing to believe that supernatural evolution operates on different rules from natural evolution, but so far the text hasn't put forth any evidence for that, or any clues about what those different rules are. Throughout Giles' entire lecture about Twilight, he continues to make comparisons to the life cycles of our own world. In context, he sure sounds like the guy with the answers, because the Universe seems to be reacting exactly as he says it will, but those of us who aren't currently dealing with Rossby waves and tears in reality are still trying to fill in the logical leaps. When Giles asked, "And do you understand how quickly the ancient demons and magicks will come pouring back in?" I answered, "No," but expected he'd get back to it.
He did, in the next issue: "There is no birth without blood....When you're born into the world, you vacate the old one. It becomes...utterly expendable." He's referring of course to the act of giving birth, in which a baby vacates its mother's womb, and then the placenta turns around and morphs into an army of demons and devours the mother, because she's expendable.
Giles, can you pick another example, please?
Unfortunately, he can't. If there is a place where humans exist in any form before they're formed in the womb, we know nothing about it. If there's another kind of birth, in which the 'old world' really does become expendable, we know nothing about that either. No wonder Twilight was considered a Watchers' myth. The kind of supposition that it would take to predict that the world would end if a new one is born doesn't fit into anyone's science or supernatural laws. If this is "an unprovable hypothesis based on theories that seek to explain the beginning of time", what's giving it enough weight to scare the Council into suicide?
Buffy and Angel were "tested" by the Universe and are being given a "reward" for being the best of their kinds, but none of the characters have yet spoken up and objected to that terminology being applied to science. Maybe they're just so used to their world being affected by supernatural entities that they can't see a line anymore, I don't know. But Giles of all people should know that the destruction of one world for the sake of a new one isn't the work of Nature; it's the work of a god.
You may have already noticed that the idea of the human race being a final product works out fine from a religious standpoint. Depending on the religion, Buffy and Angel as the first of a new kind might work out fine too. Without saying anything about my own beliefs, I'll just note that this is exactly the can of worms that I don't want to see the Buffyverse open up. So we have a new pair of gods, sure-- but if their deification means the end of our world, then whoever did it is our enemy, and the only two suspects we've got right now are Nature and an omnipotent God. Hell, the only third suspect that's even possible is a combo Nature/God, if you want to consider that as something distinct from the others. But now Nature has broken its own laws, or God is relying on a process of natural selection that automatically negates omnipotence.