One problem today is that social Darwinism remains to some degree implicit in the general free-market doctrine (perhaps more so today than thirty years ago), even though it has been long since discredited. So I see "amoral rationality" as problematic not just on account of its patent inaccuracy, but its implications as well.
I think the anthropologists and behavioral economists already have the makings of a view of humanity that would stand up pretty well in reality. But social Darwinism, even when only implicit, is an extremely comfortable world view if you're at the top of the income pyramid. So as a practical matter, "amoral rationality" will probably be a difficult doctrine to dislodge. The fact is that what we're experiencing right now is a top-down disaster. -Paul Krugman
A basic finding of social-psychology is when a member of a group - even an ad-hoc group - thinks they are out of touch with the group that person experiences anxiety, seeks to relieve that anxiety.
Could it be Social Darwinism is a coping/validating mechanism for the Ruling Class?