Starting this off, let me say that I never quite understood the rhetoric of "destroying worldview X with one argument". Not even getting into why (in this specific case) atheism is not exactly a worldview, it is an illusion to think that a person's views are so easily shattered. People adhere to their views for many reasons, the fewest of which are rational. Moreover, our worldview is something akin to a mental home. Throwing it away can cause severe discomfort to most people. So you'd have to do a lot more than pointing out one logical fallacy in order to shake the views of more than a small handful of people.
Now, let me agree with you: Yes, the fact that we have no absolute foundation for human morality is a severe problem. It means that we cannot rely on each other to share the same morality. It means that my neighbour might find it acceptable to beat his wife. It means that, beside the threat of punishment, I have no strictly rational means of convincing him that his behaviour is wrong. The only hope I have is that he has access to his basic human faculties of empathy and reason, and that I can get him to see how his behaviour violates those.
I find it interesting, and somewhat unsettling, that so many people seem to have a very specific set of demands towards a worldview: that it provide them with a sense of meaning, that it provide them with a foundation for their morality, that it tell them that human life has intrinsic value.
I believe that this is putting the cart before the horse. My worldview, quite literally, is how I see the world. And the one prime directive in choosing how I see the world is... whether this is in accordance with the facts. Can I show that there is intrinsic meaning to human life? Can I show that domestic violence is absolutely wrong? If so, then it is the case that these absolutes do exist. If not, they don't.
We don't seem to be able to show that an absolute foundation for morality (not to be confused with objective morality; those are two very distinct concepts!) actually exists. So we have to assume that it doesn't, regardless of whether this suits our tastes and titillates our religious sensibilities or not. We have to be guided by facts, not by assumptions.
And then we have to work from that.