It seems like your answer to the argument from marginal cases is that maybe the (human) marginal cases don't matter and "Making this distinction among humans, however, would be incredibly socially destructive."

That may work for you, but I think it doesn't work for the vast majority of people who don't count animals as morally relevant. You are "very sure than an 8 year old human counts morally" (intrinsically, by which I mean "not just because doing otherwise would be socially destructive). I'm not sure if you think 3 year old humans count (intrinsically), but I'm sure that almost everyone does. I know that they count these humans intrinsically (and not just to avoid social destruction), because in fact most people do make these distinctions among humans: For example, median opinion in the US seems to be that humans start counting sometime in the second trimester.

Given this, it's entirely reasonable to try to figure out what quality makes things count morally, and if you (a) care intrinsically about 3 year old humans (or 1 year old or minus 2 months old or whatever), and (b) find that chickens (or whatever) have more of this quality than 3 year old humans, you should care about chickens.