yeah, my model of the "us before them" question is that it is almost always globally optimal to cooperate, once a certain level of economic productivity is present. The safety that people are worried about is guaranteed not by maximizing their wealth but by minimizing their chances of death/starvation/conquest. Up to a point this means being strong and subjugating your neighbor (cf most of antiquity?) but eventually it means collaborating with them and including them in your "tribe" and extending your protection to them. I have no respect for anyone who argues to undo this, which is I think basically the ethos of the trump movement: by convincing everyone that they are under threat, they get people to turn on those that are actually working in concert with them (in order to enrich/empower themselves). It is a schema problem: we are so very very far away from an us vs. them world that it requires delusions to believe.
(...that said, progressivism has largely failed in dispelling this delusion. It is far too easy to feel as though progressivism/liberalism exists to prop up power hierarchies and economic disparities because in many ways it does, or has been co-opted to do that. I think on net it does not, but it should be much more cut-and-dry than it is. For that to be the case progressivism would need to find a way to effectively turn on its parasites, that is, rent-extracting capitalism and status-extracting moral elitism).
re: the first part of your reply. I sorta agree but I do think people do more extrapolation than you're saying on their own. The extrapolation is largely based on pattern-matching to known things: we have a rich literature (in the news, in art, in personal experience and storytelling) of failure modes of societies, which includes all kinds of examples of people inventing new moral rationalizations for things and using them to disregard personal morality. I think when people are extrapolating rationalists' ideas to find things that creep them out, they're largely pattern-matching to arguments they've seen in other places. It's not just that they're unknowns. And those arguments are, well, real arguments that require addressing.
And yeah, there are plenty of examples of people being afraid of things that today we think they should not have been afraid of. I tend to think that that's just how things go: it is the arc of social progress to figure out how to change things from unknown+frightening to known+benign. I won't fault anyone for being afraid of something they don't understand, but I will fault them for not being open-minded about it or being unempathetic or being cruel or not giving people chances to prove themselves.
All of this is rendered much more opaque and confusing by the fact that everyone places way too much stock in words, though. (e.g. the OP I was replying to who was taking all these criticisms of the rationalists at face-value). IMO this is a major trend that fucks royally with our ability as a society to make moral progress: we have come to believe that words supplant emotional intuition in a way that wrecks out ability to actually understand what people are upset about (I like to blame this trend for much of the modern political polarization). A small example of this is a case that I think everyone has experienced, which is a person discounting their own sense of creepiness from somebody else because they can't come up with a good reason to explain it and it feels unfair to treat someone coldly on a hunch. That should never have been possible: everyone should be trusting their hunches.
(which may seem to conflict with my preceding paragraph... should you trust your hunches or give people the chance to prove themselves? well, it's complicated, but it also really depends on what the result is. Avoiding someone personally because they creep you out is always fine, but banning their way of life when it doesn't affect you at all or directly harm anyone is certainly not.)